Check out the Buy/Sell/Trade forums right here, or use someone reputable like Horizon Wireless.
eBay is an option, but it's getting almost impossible to avoid the scams there now. That place is ruined. Best bet would be to buy here on the BBF and make sure it's a member with a lot of posts. It's not foolproof but you can have some confidence that a long-time member is not a scammer. Avoid any deal that looks too good to be true, especially if it's from a new member with little or no prior dealings. Take a look at this guide
first, especially if you use eBay.