Scam prevention

Unfortunately expensive items attract scammers, and BJDs are no exception. Within the hobby we deal with roughly 3 types of scam.

Scammers
These are people who put a doll or accessories up for sale but never send them and then disappear with the money. Online communities often place warnings when they notice such people.

Signals; Bare sale photos, sometimes even photos from the original makers, prices that are just too good, murky sales history.

Prevention; Google the seller, see if you can trace the photos by using tineye for example, ask for photos that proof that the seller actually has the item (like one with the doll and a handwritten note with, for example, a date), do not use the PayPal "gift" option.

Recasters
Recasters make illegal copies of existing dolls. This mainly affects the resin side of the hobby because resin pouring is relatively easy, and vinyl has a more complex production process.
Aside from copyright violations this practice also takes business away from the actual doll makers, whose companies generally consist of less than 10 people. Finally these dolls are made out of cheaper materials and there have been multiple stories of them breaking, or even being delivered with dust on the doll.

Statements from BJD artists regarding recasts - Den of Angels BJD Wiki [EN]
Luts article about the differences between a legit and recast doll: [\[INFO\]Difference between original & recast doll] [EN]

Signals; Seller has an unbelievably wide range of dolls, prices are well below the maker's price, "cheap" packaging that doesn't match the maker's, lack of COA or a COA that doesn't match the maker's (beware, especially older dolls were sold without COA, so this isn't foolproof). Some second hand sellers will put "Recast", "(R)" or "(RC)" in their ad to indicate the product is a recast.

Prevention; If it's too good to be true, it generally isn't true. Look for the doll's original maker so you can compare prices and verify official dealers.
If it concerns a secondhand article you can try to determine if the doll was produced pre-COA; in that case you can ask the seller to provide a different proof of purchase, such as an order-confirmation or sale-history on a forum.

Scalpers
Technically this isn't an illegal practice, but it's often seen as a form of scamming. This process is the same as ticket-reselling; Scalpers will buy as many of a Limited Edition doll as they can, as fast as they can, and then resell them for a heavily increased price.

Signals; Product with a limited number being sold shortly after the original sales period, often product photos that are straight from the original sale, heavily inflated price compared to the original sale.

Prevention; Consider how much this is worth to you. In principle this practice is perfectly legal (in the Netherlands the seller would have to pay tax over the profits made, but that's it). Generally even rare dolls will appear on the secondhand market sooner or later, so a little patience can go a long way.