I sell more on
Amazon than on Ebay. For most books, Ebay is basically a wholesale market--I can get 2 to 4 times as much for most titles on Amazon than I can get on Ebay.
The main disadvantage to Amazon is that you can only list books with an ISBN number in the Amazon Marketplace. If, like me, you sell mostly old, out-of-print books, this is a problem. If you are just selling books you have around the house, it probably won't be a problem.
Also, books on Amazon may stay listed for months or years before they are sold. Luckily, listing is free, but when you are used to the almost-instant-gratification of Ebay, waiting for a buyer can be a pain.
The shipping reimbursement on Amazon is quite low ($2.26 for Media Mail), so you can lose money on heavy books if you're not careful.
Fees are rather high (15% plus 99 cents) for each book sold so you have to figure this into your price.
Toystoryduo, Amazon has stock photos of books--you don't have to upload photos if you are selling on Amazon Marketplace. If your book has a different cover than the stock photo, you should mention this in your description.
I buy my mailing supplies in bulk online--I have not found a cheap place to buy padded envelopes, bubble wrap, etc. locally.
Ebay is planning to shut down Half.com this summer, so I would not put a lot of energy into listing on Half.
Re: penny sellers, if you sell more than 40 books a month, it's worthwhile to join Amazon's Proseller program. This costs $40 per month but the 99 cent per book fee is waived, so you only pay a 15% commission. I'm sure most penny sellers must be ProMerchants, but the profit per book would still be tiny or non-existant. Personally, I don't sell any books for less than 9.99 on either Amazon or Ebay--just not worth it.