I almost never buy anything used except on eBay. Prices are generally ridiculously high but some deals can still be found.
eBay is the current safest place for buyers. Not so much for sellers though, cuz even good & legitimate sellers can get scammed easily there.
But for honest buyers its a great place to buy because if you really do have a legitimate issue and can't work it out with the seller, you will usually get your money back.