IMO, the auction sites are a horrible, terrible and inappropriate place to collect - as in, really collect. But, it's a fine place to dabble.
First, auction seller run the range from good to horrible, so buyer beware. I buy a lot of used bike stuff this way and it's mostly good and occasionally bad in terms of the seller misrepresenting stuff. Unless I'm very, very sure of something, I try to limit my expenditures there to small purchases so if they go totally sideways, I can just walk away with minimal loss.
Second, even with a good seller, it's really hard to get a feel for what you're buying with crappy pictures and text descriptions. There's always a bit of a roll of the dice. Some are great. Some not so great.
Third, there *are* opportunities from real steals out there. If you know more about the product than the seller, you can sometimes find mis-described items that are missed by other collectors and by the seller and you can walk away with good find for little money. Digital archives of catalogs are your friend here. I (am totally nuts and) download digital scans of old catalogs of bikes and knives and drift off to sleep at night by reading them on my iPod. It's a great way to "collect" for free in that the bits are free and I get peruse my "collection" of catalog scans. Great fun and it arms you with information. Collectors guides are a hard copy variant of this.
Fourth, there are really awful knife "mechanics" out there who regularly "repair" old knives by retipping broken blades, reblading old frames and otherwise fixing up old junkers and selling them as original. There is a whole sub-culture in the forums devoted entirely to spotting fakes on the auction sites and there's just noway I'm ever going to get good enough to do that.
All this taken together, my approach is to a) stick to brands I love enough to research and b) limit my prices to a number that I can live with if I never got the knife in the mail or it was a total throw-away looser. The brands I look at are Ulster, Old Timer (prices heading north) and Buck. My personal limit for an auction site purchase is a dirt-bag low $30 (including shipping).