It really depends on what you're looking to buy, your level of knowledge, and your level of skepticism. Buying current models can be reasonably safe if you stick with reputable eBay dealers. Poorfish comes to mine, but there are a number of others as well. Buying from individuals, even if they are "powersellers" can be more risky, unless you know your Spydercos very well and know how to spot a factory second. Generally speaking, for older knives the safest bet is someone in the US or Canada with LESS THAN 500 feedbacks, and a 99% or better positive feedback score. Even then, you need to know what the knife you're looking at is suppose to look like.
For new knives from dealers, stock photos in the auction listing are ok, but for knives from private parties, avoid auctions which use them, those with poor quality photographs, and those with photographs of the left side only since Spyderco always notches factory seconds on the right side.
THG wrote:If you buy off ebay, just don't expect the knife you get to be as good as the one you saw in the pics.
Not at all sure I'd agree with that. I've purchased a considerable number of older Spydercos on eBay and, while a few did look worse, IMHO almost all of them were properly represented. A few were even better than I expected because what I was forced to assume were scratches when looking at the photos must have only been "foreign matter".