The best advice I can give you is to look at the feedback ratings, read the neutral or negative ones to see what the complaint is, and just look over what they are offering. If it seems questionable, I would look elsewhere.
There are probably a lot of honest sellers on ebay, but that still won't keep you from getting pins that are suspect. I just bought a lot of 100 from a seller that lives very close to WDW, and supposedly all of their pins came from trades with CMs at the parks. I don't doubt what they are saying, but still out of that 100 pins (actually got 102) there are two or three I feel pretty sure are scrappers or counterfeit, and about 10-15 more that I am not too sure about.
I still feel good about the seller because I know that people trade pins in the parks with the CMs and that is no guarantee you are getting "legitimate" Disney pins because if it has the backstamp, the CM will trade with you because they typically won't know if it is a fake. It is often hard to tell even if you have two of the same pin side by side.
Before we go to WDW next month, I plan to do as much research as I can about any questionable pins, and if I feel they are not legit, I won't take with us for trading because I don't want to perpetuate the problem. Some I just can't tell so I am not going to let it ruin my trading fun. It has the Disney backstamp, which is what typically is required for trade with the CMs, so if I have no legitimate reason to weed it out, we will use it for trading. I am sure we will also get a certain number of scrappers/counterfeit pins in return that will come home with us.
Still, even if I only end up with 80 of the 100 pins I bought on ebay being legit, that still means I paid about $2 each for them, which is a whole lot better off than I was last year buying sets and lanyards at the parks for all five of us to get started in this hobby at $30-$40 a pop, and then buying more because they got traded so fast.
Again, have fun with it and just assume there is some element of risk involved (some intentional and some not), and be willing to accept that risk exists on ebay if you shop there. You can only take so many precautions. If you get duped on a few, just make them keepers so you have more in your personal collection or use them to make some neat Disney crafts for your home or friends.