That's exactly it. As with fake bags and shoes unless you have the real thing to compare or really know you stuff you might never know.Be careful with OPI too! In my less knowledgeable days I bought an OPI polish off EBay and at the time I was none the wiser but now I have bought into OPI I realise that this OPI polish is 100% fake! Smells wrong and colour is wrong completely to what it's meant to be. It was a very convincing copy and if I wasn't in this business now I wouldn't have had a clue.
There are tons of fake designer handbag and shoe sellers on ebay with 100% positive feedback. People just don't know the difference until it falls apart a few months later and they just think its not worth the extra money! Such a shame because they will never know the true quality of the real item. Same for nail products.
As ever, if something looks too good to be true, it probably is. Surely it is impossible for someone to regularly sell products under trade price??? What would be their source?