The amounts I use here are just rounded numbers because I'm at work, but..
Something that I always do is I always pick up powder anywhere that sells it for the lower prices.
Cherkassk and Azaq Kale or whatever carry 4-6 of them each usually, everytime goods reset, but a lot of other towns/cities will carry 1 or 2 as well.
I don't usually send these in caravans. I'll do the other types of goods in caravans, like butter, salt, tools.
These are buyable for like 150-200 each and I never sell them for less than 800 because there are enough towns that offer this, but there are often towns that go over 900 and sometimes over 1000. Because I don't sell them for under 800 if you see some place has them for 500-600 its still profitable to buy them.
I made a lot of money doing this while I was doing quests to get my renown high enough to do the black mace quest as a neutral. I was moving caravans and horse herds all over and I would stop at every fortress and city to check prices.
Sometimes if I know I am going to Cherkassk and Azaq Kale to get more, I'll send a caravan there, and just accompany it for more money.
Another easy way to make money is to go to the villages and buy fresh beef. In the cities and forts they sell for 100-200 and they're usually buyable for 20 or so. They always sell high because they never have them in stock.
If I stop, I also ask if they have any cattle for sale, buy 5 and then slaughter them (usually the scaled up cost of the next ones to buy isn't really worth it).
Each slaughtered cattle gives you 2 fresh beefs and each cattle costs you 30-70 to buy which is always a profit of quite a lot, especially if you stop in a village close to the city you're going to so you don't have any spoilage.
If you want to make a lot of money fast, take one of those "move my cattle herd to a city."
The herds have like 80 head for me right now and if you slaughter them all that is a lot of money, there are consequences, but..you could make 25k off of that one quest depending on the herd size and if you don't slaughter them all at once (so the meat doesn't spoil), and move to different cities to keep the sale price high.