Yes this system is silly. If they have like 50+ of a resource they simply do not want any more, regardless of how quickly they use it or whether they have their own source or not. Even more ridiculous is you can sell them 1000 of a resource at full price as long as you do it all at once while they have none. I imagine it's easily exploitable, say an AI has a lot of money, but have ~200 of a resource you yourself have a lot of extras in, buy their 200 for cheap, now they have none, sell them 2000 at once for crazy high prices.
A system where if they have an abundance they are less interested in buying more makes sense, but it has to actually look at important factors like game stage, number of sources, and then be calculated dynamically rather than all at once at whatever the current price is. It should never be totally worthless, there should be minimum and maximum values.
