The part being copied would have to be something that is unavailable otherwise and/or very costly to be worth the time/effort to counterfeit it with a 3D printer. It would have to be something for which the market is very small but very willing to pay, because if the market were large, 3D printing wouldn't make sense- you'd fabricate the counterfeit in a way that's more cost effective for producing large quantities (and would probably give a higher quality result).
Maybe parts for exotic sports cars? But who isn't going to inspect and quickly know they're looking at a fake? What exotic sports car mechanic is going to risk his reputation by buying and installing counterfeit parts?
Jewelry? Too much scrutiny applied there, too.
Nope. Anything that costs a lot is going to be scrutinized. Anything that doesn't cost a lot isn't worth counterfeiting, especially not with a 3D printer.