Well, I'm still waiting to get my hands on the game, but I figured out how the WM works and it seems fine.
The whole point is that you actually give your resources BEFORE getting something. This creates a "pool" (not an unlimited one) of available goods and, provided the bid you offered is acceptable, you get what you need. This works with a stock exchange logic and probably manages quite well the setting of price and availability of the basic commodities.
For the objections risen, I would answer (just out of the data I read):
1) Germany actually has a BIG resource problem, at least until manages to get advanced conversion techs. Just consider the IC production: to get 300 IC working (this means less than complete effectiveness), she needs 600 coal (about half of dayly production) and 150 rubber (this means other 450 coal, exchanged at a 3:1 ratio). The only abundant resource, coal, is thus almost completely expended for keeping the industry running at a decent level in PEACETIME. Oh, and we didn't take into account the oil and that little steel shortage. Come on, it is apparent that the large stockpiles are the result of a very limited production.
2) Japan has an even larger resource problem, since it has nothing to give for the needed materials - notr even the German coal. So, conquest is imperative in any way.
3) The convoy system is VITAL, for UK as well as for Japan, since the resources produced overseas have to be brought back home, to use or to trade them. And troops overseas must be suppllied from the motherland, as the stockpiles outside the home provinces are limited. Subs can really do something, I guess.
4) USA has a great economic power, even greater than IRL. I mean, they produce so much oil that they can get all the rubber they need simply converting it. And they can put all the excess coal they have (which is about 400 with the industry at full capacity) on the market and trad it with rubber at 4:1, just to take 100 units off the pool and hamper Germany quite a lot.
For me, it seems to work fine (but I repeat, I'm just guessing).