Although there are 4 resource types available, due to the conversion possibilities, you can reduce it to basically 2 resources: coal and steel.
Your economy needs 2 coal, 1 steel and ½ rubber for every IC per day to function at full capacity. Furthermore, some (actually most) of your armed forces use oil. Planes and ships need oil only when moving, tanks, mech and mot inf. also when not moving (and considerably more when moving).
Most countries will need to trade for resources to keep their economy floating. When you set up a trade at the world market you are giving the quantity of a certain resource to the world market and hope you get something back. When you set up a trade 50 coal for 50 steel, you will receive 50 steel in case they are available; if not, you will not spend anything and not gain anything. You need to check how much of a resource you need for your economy and set up the trades accordingly. Usually coal and steel is available in larger quantities, so 1:1 trades are possible. The rubber shortage starts after a couple of months in the 36 scenario. From this point onwards, you will not be able to trade at a 1:1 and not even on a 2:1 ratio. Since trades are prioritised first by ratio (a 4:1 trade will be fulfilled before a 3:1 trade) and then by the time when the trade started (a trade from Jan 1st 36 will be fulfilled before a trade from March 36), it is also unlikely that you receive rubber on a 3:1 trade when you set up the trade too late. Please note that the AI only sets up trades till 3:1, so a 4:1 trade will always prioritise you. So it is a good idea to start trades for rubber (if you need it) at the very beginning at 3:1 for the amount of rubber you need (adding a small surplus) and throw in some 1:1 trades to suck off the surplus rubber in the beginning. You can keep the trades since if they are not fulfilled, you keep your coal and maybe you get some cheap rubber when, in the future, there is a surplus for a couple of days.
When making offers for oil and rubber, consider your conversion techs. For the USA, there is basically no point in spending 3 oil for 1 rubber since they can convert it at 2:1. Same is true for Germany as soon as they have discovered improved conversion techs where it needs 4 coal to convert (via oil) to 1 rubber.
There is an exception to this, when you want to stockpile all resource types, it makes sense as USA to trade for rubber. But why would you want to do it? Especially as USA (but also as SU), you can upgrade your ICs at the beginning for relatively low costs (due to the high consumer goods demands which will slowly decrease). So you can easily end up with more ICs than you have resources for (especially with the IC increasing techs). So having some stockpiles will help you extend the time when your economy can run at it’s fullest (and since you cannot hold more than 99.999 of a resource in your capital stock, the excess is lost, so you can trade this surplus at any rate for a resource you need). To create additional stockpiles, you need to set up convoys from your homeland to an island or any part of your territory which does not have a land connection to your capital. In each of this provinces, you will be able to stock 9.999 of coal/rubber/steel. When you need the resources, you can ship them back to your homeland.
As soon as the big war starts, you should cancel all your trades since the efficiency will be between 10% and 15%, so for 300 coal, you will not get 100 rubber but between 10 and 15. If you need steel, you might have to trade even with this bad rates. If you are waging an early small (and relatively quick) war, you should keep the 3:1 trades for rubber, since it might be possible that after the war, you will not get a 3:1 trade anymore (due to the prioritising).
When there is 0 of a certain resource in the market, you need to overbid (ratio wise) other countries. This means maybe a 2:1 (for coal:steel) 3:1 (for coal: oil) and 4:1 (for coal:rubber). It might be also 3:1 or 4:1 for the coal vs. steel/oil. When your trade is displayed in red letters, this means you get nothing from it and need to make a better offer.
Independently from the country trades there are resources added to the world market (thx to Steel for this info):
200 Oil
600 Rubber
5 Coal
5 Steel
This means that you can for example always buy at least 200 oil per day (as long as you are willing to outbid the AI) even if nobody sells oil.
Your economy needs 2 coal, 1 steel and ½ rubber for every IC per day to function at full capacity. Furthermore, some (actually most) of your armed forces use oil. Planes and ships need oil only when moving, tanks, mech and mot inf. also when not moving (and considerably more when moving).
Most countries will need to trade for resources to keep their economy floating. When you set up a trade at the world market you are giving the quantity of a certain resource to the world market and hope you get something back. When you set up a trade 50 coal for 50 steel, you will receive 50 steel in case they are available; if not, you will not spend anything and not gain anything. You need to check how much of a resource you need for your economy and set up the trades accordingly. Usually coal and steel is available in larger quantities, so 1:1 trades are possible. The rubber shortage starts after a couple of months in the 36 scenario. From this point onwards, you will not be able to trade at a 1:1 and not even on a 2:1 ratio. Since trades are prioritised first by ratio (a 4:1 trade will be fulfilled before a 3:1 trade) and then by the time when the trade started (a trade from Jan 1st 36 will be fulfilled before a trade from March 36), it is also unlikely that you receive rubber on a 3:1 trade when you set up the trade too late. Please note that the AI only sets up trades till 3:1, so a 4:1 trade will always prioritise you. So it is a good idea to start trades for rubber (if you need it) at the very beginning at 3:1 for the amount of rubber you need (adding a small surplus) and throw in some 1:1 trades to suck off the surplus rubber in the beginning. You can keep the trades since if they are not fulfilled, you keep your coal and maybe you get some cheap rubber when, in the future, there is a surplus for a couple of days.
When making offers for oil and rubber, consider your conversion techs. For the USA, there is basically no point in spending 3 oil for 1 rubber since they can convert it at 2:1. Same is true for Germany as soon as they have discovered improved conversion techs where it needs 4 coal to convert (via oil) to 1 rubber.
There is an exception to this, when you want to stockpile all resource types, it makes sense as USA to trade for rubber. But why would you want to do it? Especially as USA (but also as SU), you can upgrade your ICs at the beginning for relatively low costs (due to the high consumer goods demands which will slowly decrease). So you can easily end up with more ICs than you have resources for (especially with the IC increasing techs). So having some stockpiles will help you extend the time when your economy can run at it’s fullest (and since you cannot hold more than 99.999 of a resource in your capital stock, the excess is lost, so you can trade this surplus at any rate for a resource you need). To create additional stockpiles, you need to set up convoys from your homeland to an island or any part of your territory which does not have a land connection to your capital. In each of this provinces, you will be able to stock 9.999 of coal/rubber/steel. When you need the resources, you can ship them back to your homeland.
As soon as the big war starts, you should cancel all your trades since the efficiency will be between 10% and 15%, so for 300 coal, you will not get 100 rubber but between 10 and 15. If you need steel, you might have to trade even with this bad rates. If you are waging an early small (and relatively quick) war, you should keep the 3:1 trades for rubber, since it might be possible that after the war, you will not get a 3:1 trade anymore (due to the prioritising).
When there is 0 of a certain resource in the market, you need to overbid (ratio wise) other countries. This means maybe a 2:1 (for coal:steel) 3:1 (for coal: oil) and 4:1 (for coal:rubber). It might be also 3:1 or 4:1 for the coal vs. steel/oil. When your trade is displayed in red letters, this means you get nothing from it and need to make a better offer.
Independently from the country trades there are resources added to the world market (thx to Steel for this info):
200 Oil
600 Rubber
5 Coal
5 Steel
This means that you can for example always buy at least 200 oil per day (as long as you are willing to outbid the AI) even if nobody sells oil.