General rules of thumb that I follow:
-No listings from China, especially if they're super low
-As mentioned above, nothing from a seller listing the same thing over and over- especially rare pins, and especially if super low
-No large lots. It's possible to find authentic large lots, but as a general rule, I don't do them
-If it's too good to be true, it usually is
-Backing cards and things like original boxes for mystery pins are added bonuses. It's not always a make/break decision for me (especially with older pins), but it helps.
For authentic large lots, I'd look here first, on the selling page. Then I'd look at people selling off a collection, things like that. Or someone with a small seller score (small as in "they don't list often", not small as in negative reviews). Thought process there is that the people with the large seller numbers are usually selling lots over and over again. But even buying like that is risky, because a casual pin fan might not know their stuff is scrappers.
Basically, you'll start to notice who's selling the scrappers and bootlegs. Once you weed those sellers out, most of the rest of the sellers are fine. You just have to determine if they're overpriced or not.
And be careful of reviews. There's lots of scrapper sells with amazing scores, because a lot of the people who buy from them either don't know they're buying fakes, or don't care- either way, they tend to leave good scores.
