Hi! First of all, welcome!
Your question is one with many possible answers! There are both legit and non-legit ways that secondary sellers (people other than Disney) can have pins for sale for cheap. Here's a few of them:
Legit
1.) They're employees, and get cast discounts (or can buy from the discounted cast store once they go clearance).
Technically, they aren't supposed to re-sell, I think, but a lot of cast members (and their friends and family) do so anyway.
2.)Outlets and clearance sales
Occasionally, Disney will dump pins at outlets or have clearance sales. This used to be much more widespread than it is now...the selection has slimmed down a lot, and cast member stores like Company D get most of the clearance pins nowadays.
3.) Dumping collections
This won't be new releases and sealed packs, typically. But sometimes people sell off things they bought in estate sales and the like for cheap. Still legit pins, acquired by legit means...but usually older pins, and they won't be able to replenish.
Non-Legit
1.) Theft
People go into the parks with empty totes and just steal them, then sell them for half price.
2.) Warehouses/Factories in China
Similar to scrappers, the sellers in China have figured out that they can take product that's new on card, then sell it directly to buyers over here (whether it's to the pin collectors directly through ebay/facebook, or stores). Either way, they don't pay for them and aren't authorized to take them, so they're basically stealing them.
Re: #2 on the non-legit list...there are a lot of stores (both online and brick and mortar) that will tell you they get Disney's "overstock" from China and can buy them off them for cheap. I seriously doubt this is the case...especially for product that Disney is currently selling on the rack in the Parks. I think it's much more likely that someone in China is running extra off the runs, or they're just straight-up stealing the shipments. Unlike scrappers, these are pretty much the official pins, in package or on cards, so it's very hard sometimes to tell what was stolen from Disney, and what's not.
My rules of thumb when buying pins that are deeply discounted:
1.) Does the source seem trustworthy?
2.) Can they track the pins history (or does it make sense why it's so cheap)?
3.) Don't buy from China (unless it's a known and respected pin trader)
4.) Don't buy from someone who offers up lot after lot and sells in bulk
And the golden rule...if it's too good to be true, it usually is. This is especially true of LE pins...it's pretty obvious that when a gazillion copies of a DSF/WDI pin suddenly shows up for $5, that that pin has been counterfeited. In that case, be careful if you're looking to buy or trade for it. Sometimes, they people who end up with them don't know that what they've got is a counterfeit.
