+1 on reverb.com. I have a store there and have had nothing but good experiences. They make it very easy for a first time seller. Also, it isn't an auction site, so you don't have to worry about no one bidding and your guitar or bass selling for less than you want. You can either just list it at a fixed price, or list an asking price while entertaining offers. Unlike eBay, if you don't like the offer, you can just reject it or counter-offer. When you sell, they will offer you a discounted shipping rate with UPS and let you download the shipping label.
Bill, tgo