eBay is free to list items up to 50 a month now. They collect a 10% final-value fee when you sell an item. Then you have to get your money out through PayPal, who takes another cut. You also need to consider packaging and shipping cost. Amazon takes differing fees depending on category, but it usually works out to about 15-20% of the selling price.
Neither eBay nor Amazon are things you just jump into without doing a fair bit of reading first. And please make sure you understand the terms of service. The number of clueless Amazon sellers in the Amazon forums is preposterous. Amazon is much more strict then eBay. For example, you can't sell items with any parts missing (even redundant or useless ones). What you are selling must match the item description exactly. Putting details to the contrary in the listing notes is against the TOS. A lot of sellers don't know that.
For example, if you list on the Amazon page for a Widget, color Red; you can't then say "mine is actually blue." This doesn't stop sellers from doing it all the time. If a buyer complains, however, the seller will lose the A-Z claim.
