I've made over 60 satisfactory purchases on eBay. One turned out to be not as described. When I questioned the seller, he took the item back and refunded my money promptly with no hassles.
As with any market, the buyer has to be careful. I frequently see items sell on eBay for more than you can buy them new at retail from any number of well-known suppliers. I even see common, current-production stuff readily available sell for over MSRP on eBay. Stupid buyers.
I also sometimes see things listed incorrectly on eBay. For example, just a few weeks ago I corrected a seller who had listed a Benchmade Model 42. The picture and the description were of a Benchmade 31. When I brought it to the seller's attention, he did not huff off vowing never to sell on eBay again. Instead, he thanked me and corrected his listing. And why not? He's more likely to attract a bid and less likely to have problems after the sale if he lists the item correctly.
I won an auction just the other day (not a knife) and was floored to get an invoice for $30 when the winning bid was cents over $15. It seems that this seller charges a bit of a fee for packing. I missed that in the fine print of the listing. My mistake. I learned a lesson.
In general, don't buy something on eBay if you're not sure about what you're bidding on (unless it's really, really cheap and you're willing to gamble). And read the whole listing. Be sure to check where the seller is located. I once won a balisong knife only to find out that the seller was in England. It's illegal to import a balisong into the US so that one involved a risk of a loss to customs (it turned out that the knife came through just fine). Also, international can mean long shipping times, higher shipping costs, and you may get slapped with duty. And check out the seller's feedback. Finally, check out the seller's previous auctions to see if the seller routinely sells this type of item. If not, there's a higher chance of errors in the listing.
Basically, be a little bit careful and you can get great bargains.