It's Amazon Arbitrage. I have read about people also selling things on yahoo auctions that were purchased on ebay, and vice versa.

Amazon is obiously going to be allot more stable to deal with because the prices should not change as much.

If you have a hard time dealing with Amazon just have them deliver to you, and then send to the customer (Maybe in a new package). Actually touching the product will also allow you to upsell the customer by including a list of more products you are selling.

Why don't you also start buying through an affiliate link to get a few extra change.