I like the general idea, but this could be abused really, really, easily. All you would have to do is create 2 private chest shops, and then have a friend buy and sell items for a ridiculous price, therefore being able to keep all of his money, and give the shop owner lots of money.
Example: If the investor gave you a 1% bonus....
Sell 1 Dirt 1,000,000= 1,000 Profit