I'm new to this too...here's a few tips that I can think of:
1. Don't buy "lots" of pins on ebay. There's a lot on that say "lot of 100" and you can buy it now for $55. If it's cheap, be skeptical. A pin shouldn't be under $3.
2. If you can get a LE 250 for $5, it's probably a fake.
3. There are more counterfeit pins on ebay than real ones. (Ok, I have no proof and may be wrong but just assume this anyway)
4. If you're interested in a specific pin, look it up on pinpics.com and see if there are known counterfeits. Some list details of differences to compare to the real ones.
5. Just because the seller has 2500 positive feedback, 0 negative, and sells nothing but Disney pins doesn't mean they're real. (I noticed this last night on ebay)
6. There's no real way to know unless you buy it directly from Disney.
Note: I don't ever buy pins on ebay that have known counterfeits made.