Summarize:

If you want to make the best possible impression on your customers, package the product elegantly (making sure it won't break during shipping), bring it to the post office, and ship it off. It's really as simple as that. Unfortunately, sometimes the customer isn't going to like what they bought. Make the return/exchange policy clear, but don't burn bridges by refusing a refund. Eating these refund costs will be a good business practice, and should keep your Amazon/eBay/Etsy ratings high.  Cycle the feedback you get into making your products even better. Iterate on bad designs, negative interactions, or product flaws. Remember that the customer is always right, even when they aren't. It's one of the hardest parts of doing business, but it's one of the oldest rules in the book. If you treat your customers like undeserving bums, they'll feel like that. And while that may feel good after a brutal exchange, it certainly doesn't pad the wallet. In the beginning, it can be a good strategy to stay focused on one or two products, so that you get a hang of the process and don't spend too much time juggling pictures, descriptions, demographics, etc. After you've gotten a foothold in a market and developed confidence in an e-commerce platform (such as eBay), it could be profitable to start selling distinct, but related, products. If you're serious about making money, you'll want to look at your sales after a couple months and figure out how to increase them. Here are just a few ideas you can use:  Negotiate better prices from wholesale. As you buy more in bulk, your power to negotiate goes up. Don't be afraid to use it! The wholesalers want your business. Look for recurring revenue sources. Think about ways you can ensure repeat business. Could email, snail mail, a subscription pack, or something else creative get you return business? Ask for help, or outsource. Could a few more pairs of hands and legs help you deliver more packages and increase the sales you make? Especially if you're only selling part-time, constant trips to the post office and regular time sunk into payment processing could be eating away at your profitability.
Ship the product out expeditiously. Offer refunds and exchanges. After a while, branch out to new products. Slowly but surely, start to sell bigger and better.