Go check the Collectors board- they have a list of good/bad ebay sellers.
Do NOT buy pins from sellers that are based in China
Do NOT buy pins from sellers who have "pick your own lot" offering over 300 pins total
Do NOT buy pins from sellers who have large lots (50-100 pins) for less than $1 per pin
Do NOT buy pins from sellers who have MULTIPLE large lots
Do NOT buy pins from sellers who have ANY negative feedback stating that a buyer got a scrapper. Many pin sellers will have 99% feedback, and when you look close they have had 2 or 3 buyers state they got scrappers.
Do NOT buy pins from sellers who state that the pins will come in individual baggies
Do NOT buy pins from sellers who use phrases like "100% tradeable"
Do NOT buy pins from sellers who have feedback from making purchases from pin sellers from China
LOOK for ebay sellers offering pins in original packaging and/or on original backers
LOOK for ebay sellers offering single pins for around $1.00-$2.00 a pin
LOOK for ebay sellers with NO feedback regarding scrappers
LOOK for ebay sellers based in the US
LOOK for ebay sellers on the collector's GOOD SELLERS list
Honestly, buying pins off of ebay is really risky- most of those large lots are either scrappers directly from China, or locals who buy large lots of scrappers and then take them into the parks and trade the scrappers with CMs to get "authentic park traded" pins.