I have had good luck with ebay. I have Bought bike parts, clothes, a full bike and a frame.
On big ticket items I use escrow.com. It protects both buyer and seller. Seller sends product after they are notified buyer has deposited $ with escrow.com. Money released to seller,by escrow.com, ONLY AFTER buyer examines and approves the item. There is a $35-$65, charge, but it is worth it with big ticket items, Most sellers are willing to negotiate the escrow charge, since they don't have to pay a paypal service charge