its a great place to buy, just buy from people who have done a lot of selling on there and have good feedback and you will have no worries. Only dissadvantage is that you don't get warrenty's on you purchase.
Like Savage said look for items advertised as new. Buying used electronic stuff off of ebay can be risky - especially dss stuff. Goto sounddomain.com for used car audio.
I've been selling on Ebay since its inception. I've sold car stereo stuff...I'm very detailed about the condition, but I sell it "as-is" and if there is any applicable warranty from the manufacture that is all I can assure.