Something I actually know a little about.
I am a Product Manager for a well-known computer monitor manufacturer.

Many sellers on E-Bay are actual companies (mostly small) selling their product via the internet. They are not all your "average Joe's".

They may buy discontinued stock, over-supply items, quantity discount items...all "A" stock items, that the original manufactuer wanted to sell out, or they may purchase from resellers who took too large of a stock position from the manufacturer and through their terms and conditions can not return them to the manufacturer (sometimes manufacturers only allow up to 15% of their merchandise to be returned from their distribution partners). They may also buy merchandise from brokers, who either work with the manufacturer or distribution partner...still should be "A" stock.

On the otherhand, some people also will sell "B" stock (opened-package or refurbished). They buy these through the same channels as listed above. There is usually nothing wrong with these items either. They are of "A" stock quality, but can not be sold as NEW or "A" stock by law. Some times these are the best deals...however, check if the original manufacturer's warranty still applies. (sometimes "B" stock may have a 1 year warranty, while the "A" stock has a 3 year warranty for example.)

And on the third-hand, there are some guys out there that are not doing either (they are the scum many refer to). And that's where you need to be careful. I have heard that you should take into consideration their ratings and that the most secure way to purchase is through PayPal (NOT a certified check)! I am not an expert on this part of the equation, this is only what I have been told.

A friend's son ordered an item and paid with certified check. The seller was a scam artist and got his money...and did not send the item of course. Either Yahoo or EBay told the friend that they can not check the background of every seller...they suggested to use PayPal in the future since PayPal does the background check and knows who the seller (or buyer) is and where their bank accounts are. There is a chance for remedy in these situations.

Hope this helps.