For a good list of eBayer sellers who sell "good" traders please visit the collectors board or PM me.

I've purchased from mousepinsonline.com and been very pleased with what I've received as well. You can find pins for less than rack prices by shopping on eBay but eBay is rampant with pins that are not authorized or approved by Disney, these pins are commonly called scrappers and are sold for pennies on the dollar. I just want you to be aware that there are sellers/traders out there with "scrapped" pins that were not approved by Disney and were supposed to be thrown out (or scrapped, hence the term scrapper) or destroy but instead were sold on the secondardy market.
In the parks you can find starter sets with lanyards and pins for trading or you can find lanyards by themselves (they make small ones for smaller children) and then just buy an assortment pack for trading. Have fun trading, I'm sure your DD will love it!!