Every merchant should have a static inventory and a faked "demand vs available" script:
For example, a merchant would have a max capacity of 1000 apples, he won't buy more until someone buys his apples.
Lets imagine apples are the jewels of the wasteland, and every player would buy them on sight: if his apple stock is sold entirely in a given time (from him buying the apples to him selling them), his max capacity would increase; and the reverse would also apply: if he can not sell his apples, he will lose interest in them, so he will buy less and less, his static inventory will show that he has 1000 apples, but in fact, for example, he won't buy apples until his stock are under 10 apples.
With this it could also be interesting to add a variable price: the higher the demand the higher the price gets.
first step to introduce a real economic system. At the end it would require a more complex town per town, merchant per merchant, item per item algorithm which would make all the prices to fluctuate.