Ya I get what your saying, there bigger and have more people working with them, but if neopets only releases a certain number of say M-Skulls and if a site has an unending stock of M-skulls then obviously theirs something going on, and most likely its duped.
And like you said yourself you can never know its duped or not, you just paid money for a pile of shit literally, until TNT exposes you. Unless its HT items since they are easily accessible but some cove items could be duped. Also the nps they generate could probably come from them Autobuying.