I sell Steam Wallet Codes (gift cards) for Bitcoin. I've done over 1500 BTC in sales in the last year and make about 5% profit, depending on the current BTC/USD exchange rate.
I started the site as an experiment to build my own payment processing system and got lucky with the right product.
I spend about 5 minutes 3 times a day restocking inventory when I get an alert on my phone that stocks are low.
I've been experimenting with 0-confirmation transactions and hope to relaunch in the coming weeks with a service that emails codes in under 60 seconds.