Its more like bartering.
I wasn't planning on getting in on this, but how is it bartering if one party is not aware that they are bartering?
If he goes in and says "I have these packs that i didn't get from you, but I want to trade for credit, is that OK?" and they say "OK", then that is legitimate.
But if one side is hiding information from the other side about the origin of the goods, then please don't think that they're not doing something shady.
If it's not shady, then be honest with the store and if they're ok with it, god bless.