Problem with services like this is, the buyer will always have the disadvantage. Scammers will do everything to ruin the trade like putting just some random text and sell it for any products/codes that he/she likes , ex. activation/serial code. Your site will absolutely be use as an instrument for scams.

Anyway, this is the things that I want to suggest:
1. Lower the rate to 0.0001 - Maybe? there will be a product lower than 0.001
2. Add comment box per product or thumbs up thumbs down feature - Only those who have bought the product can put a comment/thumbs. This will serve as a rating.