I don't understand how this is different to the current system.
Also surely more stock would often be a bad thing for shops that want currency Diamonds etc not stock they can't usem

Unless they buy the item at a cheaper price then they are selling this is just a complete waste of time

The economy and shops are controlled by the players, setting a "standard" isn't going to change how the shops work. I have a huge amount of Ender pearls, so I sell them ridiculously cheap, 54 stacks for 2 daimonds is the price I would sell for. The two way exchange for a person without ender pearls and access to a grinder would be ridiculously expensive for said player, so consumers selling my product to me is out of the question. Also, if I run out of Ender pearls due to massive buying, an hour or so at the end grinder will change that.

Also, just a question:

What is the point of making a two-way exchange? For me, I'm selling something I DONT want (epearls) for something I DO want (daimonds). I don't want what I sold given back to me, at least not for the same price I sold it for.

So like the shops in Pokemon games?

if you want to sell stuff, why not make your own shop rather than trying to sell stock to the store? Unless a shop specifically hires you to help stock its safe to assume that the shop owner is ready and willing to keep stock at approprite levels on their own