Half.com is a a great place to get a used Palm. The prices are lower, and you don't get into a bidding war. I highly recommend buying a used Palm through this site.
ebay is the obvious place to look for a palm or a Treo, or a used iPhone. Unfortunately, the prices tend to be very high on eBay. The bidding is driven up by the high traffic. So, it is a good place to sell your old phone or PDA, but not such a great place to buy. It is probalby worth trying a few low bids to see if you get lucky.