another reason: Diet Coke and Mentos

Another problem with the scenario you have described:

Without prices to signal how to value various products, some guys would take all the Diet Coke and Mentos just to be able to build bigger and better displays in which both would be expended to satisfy their desire to see the chemical reactions.

Or, substitute these products for others that could be wasted in gratuitous ways. I know some of the guys who would do it if they could, and they don't give a damn if it would inconvenience someone else, if it would be fun for them.

Economic reality keeps them in check.