As far as e-bay goes, I've bought a number of items through there and so far have had very good luck.
On the other hand, a friend of mine bought a turntable and is going through hell because the seller was a) slow to ship, b) packed it poorly, c) underinsured it, and naturally d) the post office crunched the box and ruined the turntable (bent spindle) and a dinged base.
The guy that insured it is the payee (not my friend) and the box and contents have to be shipped to the originating post office to file the claim. He is now two months from having paid for it and is just beginning the claims process. My guess is he won't have anything for months more. I've tried to get him to demand his money back from the seller but he seems fine with just going this route.
At any rate, on ebay you are dealing with individuals (for the most part) and really have no recourse except to write a nasty feedback if they screw you. If you pay by credit card you have some protection and the seller is also generally a company then. If it is an individual, make sure you discuss shipping damage and what warranty if any will apply - can you send it back and get your money back if something doesn't work or gets cruched during shipment.
All that said, one thing to keep in mind.
You get what you pay for. Even buying from a company, if you buy the bargain basement box, you will get cheap knockoff parts, maybe relabelled RAM, little or no support, etc. Get familiar with the FCC website and how to track down manufacturers using the FCC ID on the various cards. That way you can maybe get driver updates, etc.
I've had to help friends who bought the bargain basement junk and I have quit doing it (unless she is really cute

) just because it is such a pain in the ass.
Spend a little more if you can and get good name-brand stuff. In the computer industry, it really does make a difference. Maybe not when you open the box and plug it in, but it can make a huge difference once an incompatibility shows up after installing some new program or driver.