We have had good success purchasing parts off eBay. Just make sure if there's not enough info or pictures, ask questions. You soon get a fair idea if the person knows his stuff or is just clearing the junk and the item is as described.

We brought our tandem off Trademe (NZ version of Craiglist). If you know what you are looking for and can confirm the actual product (we drove 1 1/2 hours to look at the tandem) it's well worth it.

I have also sold a bike this way and was happy with the price I got, I just made sure I was honest about the bike and realistic about what I could get for it. It sold first time!