This is an OOS change so maybe could be tested in @borg-'s balancing mod or the Com mod by @real_tabasco_sauce

Since that happened this WE I looked into it, and got a grasp of the problem. The prices are currently driven by the number of transactions and not the volumes they generate. So one can drive the price of a resource then benefit from large transactions, therefor overall generating resources. Currently the formula multiply some constants to sold amount (which is equivalent to number of transactions, as sold amounts are fix), if you change it to bought amount, large transaction affect the price more, and generating resources from this 'Pump and Dump' no longer works   => The only way to have barter efficiency over 100% will now be if ANOTHER player oversold a resource, and the amount you can generate is now proportional to the amount he lost. Which is very likely the behavior intended in the first place.

See: https://gitea.wildfiregames.com/0ad/0ad/issues/8051

Under certain conditions, the market can be pumped exploited with OP trades that will generate infinite resources. Below is one example of the exploit: One needs to firstly invest 3000 metal to buy some other resources, e.g. food, to really depreciate the metal value and raise the food value. Then you take 500 food to trade back more than 3000 metal - you end with a net profit in metal. Furthermore, the food is still highly valued and you can buy huge quantities of other resources with food, resulting in resources being generated from nowhere.  However, this is not consistently working. I tried to reproduce this but did not get these profitable values like Cube did. After many trades, I was eventually able to generate 10K metal after spending 2000 food and 2500 wood (profit?)   Ideally we make changes to the bartering mechanism to prevent this from happening. 

Could be. Although if you read the story we the devs back then actually proposed a game before age of mythology

It seems to me that this system was copied from Age of mythology.