Buying Pins on Ebay

[Parisite]

My family goes to WDW every year. This year, for the first time, they bought a few pins and traded with CM's. We're not interested in collecting, but the trading adds a new dimension to the vacation. I see many pin lots on ebay for under $3 per pin. I know not to buy Sedesma or the 2000 pins. Is there anything else I should know before bidding? Are the other pins just as "good" as the ones in the park? Aer there trading issues there? What about the ones I saw at WDW with plastic backs - my daughter accidentally traded for one but I traded it back ---- how do I know how to avoid those? Thanks so much.