I would have to say Ebay and Amazon. They are both very safe to use, and you can find the right price for what you're looking for, new or used.
Both have an easy to use site and if you happen to run into any problems with your order, 99% of the time it will be solved with an exchange or refund.
Whatever you do, stay away from I-offer it's a scam artist's heaven and you never know what you're going to run into. Extremely difficult to solve any issues you might encounter, even with Paypal.
I once ordered a car stereo for $350, and was sent a box of rubber bands. It took over a year with I-offer before they agreed on giving me only $200 back. After a very long and frustrating process. Use that site with caution.