Shop Selling items only to allies?

and not to enemies?

How?

Trigger your own shops that give ownership to allies in range and not enemies?

Add the Shop Sharing ability to the shop, so it can only affect allies, and set in the field "Items Made", the items that the building can make.

Never heard of shop sharing before....
But I trust it works if you say so. +rep.

It's an ability on buildings like Arcane Vault. (Self-built item shops in melee games).

Never played melee wc3...

If you own the shop and its not neutral is one thing, another part of it as moyack stated that you need to make the ability for selecting and selling items target only you and allies.