Because no legitimate seller on ebay could make a profit selling pins at $1.00 each. The majority of those lots are filled with scrappers purchased directly from factories in China who illegally purchased the molds from original Disney pins.
Disney pins cost at least $6.00 in the park if you buy by the single pin. Between sales and the outlet store, you can get you cost down to $2-$3 per pin. Sure, occasionally a seller will thin out their collection on ebay, but who has 10,000+ pins to "thin out" of their collection? Ebay sellers are looking to make a profit. If they're selling pins at $1.00 each, if they are truly selling authentic pins, they'll be losing at least $1.25 per pin between ebay fees and paypal fees.
It's my business because people who trade these scrappers in the park ruin pin trading for the rest of us who actually buy authentic Disney pins.