I have been selling products on eBay for close to 15 years. Right now I am selling off the inventory for an online store that shut down due to hits from both Google and the economy. The costs have come down a little on eBay on the past year. You no longer have to pay insertion fees for most products, but they take a 10% cut in commission, plus PayPal fees. The main gripe I have about eBay is that they charge commissions on any shipping fees that you charge a buyer.
I have some friends who sell products as 3rd party sellers on Amazon. You can move a boatload of products on Amazon if you play by their rules, but the rules are tough and fairly expensive. I think Amazon takes about a 20% commission. It is hard to find products that you can price competitively and still make money after a 20% cut is taken out.
"Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote." -- Benjamin Franklin