On buying pins:
The first thing I should do is send you to the two following sites-
Pin Pics for information about the pins, go to pinpics.com (sorry, it wouldn't let me post the link.
For forums on collecting pins, a place to check references on pin traders go to dizpins.com.
Each of these is as useful for Disney Pins as these boards are for Disney.
I have both bought and sold pins on ebay. As others have said, check references, know your pins, ask questions. Oh, at DSL this summer, the pin thing was really low key, almost nonexistant. Don't know if WDW is the same. Also, like beanie babies and Pokemon trading cards, pin prices can be silly. Be sensible.
Good Luck.