For $5 per pin, I wouldn't worry about it, unless it's a brand new release. There are outlets, cast sales, and some Disney pins just retail (or used to retail) for around that price...especially mystery packs. I've been able to offer pins for pretty cheap before due to old SALSA sales, for example.
THAT SAID- in addition to the methods Hopemax mentioned above, there's also the new issue that seems to be popping up in places like Frank & Sons and the little pin stores around (at flea markets, etc.)- the workers in China are now offering pins on their cards (or in mystery bags) to sell in bulk. I don't know if anyone knows for sure if they're stealing them, or running off extra copies to take for themselves, but it seems like the Chinese workers are helping themselves to Disney's products in this new and terrifying way in addition to scrappers and counterfeits. Terrifying, because I'm not sure there's a way to distinguish them from the actual product.
In general, I just try and feel out the person. If there's an obvious way they could be doing it (they're a cast member, for example), then I'm good. But if they seem fishy, or answer the question about how they can afford to do this vaguely...beware.
And I never, NEVER trust new releases at that price. MAYBE if it's a cast member selling after 50% days...but that's it. There's no legit way to get a new release for rock bottom prices.
The places that do the "bulk on card" thing typically use the "I have a license with Disney, and they sell me their extras to sell for cheap" excuse. I don't buy this at all, as Disney has several ways to get rid of their extras (cast stores, outlets, etc). When they sell a license to someone, they usually make cheaper quality products for that specific retailer, or give allowance for new art (like ACME)...not hand over merchandise they sell on the racks themselves, and then allow that retailer to undercut them by half. And that ESPECIALLY doesn't make sense for brand new releases.
So...yeah.
