its beacuse they are pre-used or otherwise. In the descriptions there will be hints as to how good they are. I bought some stuff and have yet to reciev so look at ur byers feedback before bidding and beware based on they're bidding.
I have bought a head unit and a set of speakers of ebay. The head unit said it wasn't a used or remanufactured, but i believe it was. One side of it the lights don't really work all the time, its not mine anymore though so not my problem. Other than that it works fine and is good. As for the speakers that I bought i have had them for almost 1 1/2 years and they still work perfectly still hit the highs enough to make you turn it down. It really depends on who the seller is and what the price is. The speakers were like $49 on ebay and $110 in crutchfield so it just depends.
As for an amp I would probably not buy it unless it is from one of those powerseller dealers people or whatever they are called.
I have been ebaying for a year or two now, and I can tell you that asking if Ebay is any good is like asking if Steakhouses are any good (assuming you like steak).
It all matters who you buy from
Buy from a guy with a rating of 3 and 2 negative ratings is a gamble. Buying from a guy with a rating of 12000 is sometimes better than going to your local shop and getting a sub. So if you pay attention to the auction, buy from a Powerseller, make sure you know what you are getting and when to expect it, and make sure they have a return policy if they lied.
However, Ebay does also have buyer protection services, such as insurance, feedback, and etc.
I say if you find a good deal from a respectable seller, hit that buy it now button and get your ass a good deal.