Let's make it an honor system.

We get two rooms right?
You just drop your product in the item room and take how much you think it's worth from the Gil room. Then when you want to 'buy' something you take it from the item room and toss how much Gil you think it's worth in the Gil room.

I have the utmost confidence in the game community that this will work. There is no possibility of anything going wrong with this idea, ever.