I think the 200,000 maximum is fine. This is the way to maintain a balance in purchasing resources...
We must remember that market prices fluctuates base on supply and demand. it would be unfair for other players who would purchase resources 1 second behind you.
Example, by building fortress 7 you need a lot of stone which can push prices higher.
Can you imagine you can bought those stones at very cheap price, while after buying pushing the prices by several decimals?
Abuses might occur it no limit would be placed. Any one can just simply keep on selling or buying to monopolize the market so that other players will have difficulty in upgrading what ever they wanted.