If you sell a book on amazon.com for $55.00 you should come away $55.00 dollars richer right? Not if the person buying the book lives in Hong Kong! A fashionista I know sold a rare fashion book on amazon.com for $55.00 not anticipating the buyer would live outside the US. Amazon.com gave the seller a $12.99 credit for shipping fees which did not come close to covering the cost of shipping a hardback book to Hong Kong. The cheapest shipping method was $30.00! Ultimately the seller took about a $17.00 loss to her book profits simply because she didn't state in her description that she would sell to buyers living in the US only.
Even if you sell an item to a US buyer through amazon.com. the measly $3.99 shipping credit they offer for books doesn't always cover the real cost of shipping. If you sell on amazon.com, price your items to absorb some of the shipping costs.
This is just one example of how selling online can be less profitable than the seller expects. When selling on eBay look out for fees for everything including pictures of items for sale, a listing fee, a percentage of the sale and a Paypal fee if you choose to use Paypal. The more fees you pay the lower your profit. Selling on eBay makes more sense if you are selling something for hundreds of dollars that serious buyers will get into a bidding war over, but think twice before selling lower priced items whose profits could be virtually eliminated by all those fees. You deserve to make a decent profit from your stuff and you can if you avoid the traps mentioned here.