All they have to do is copy text from similar adverts.
I'm pretty clued up, and I was nearly conned when I tried to buy an S4 engine. The advert gave a lot of technical info that only someone 'in the know' would have have said.
I only twigged when the bloke said he lived on a remote Scottish island so there was no way I could see it. Turns out, after a bit of investigation, the advert text was an exact copy of a genuine one that Rich Weir had posted nearly a year earlier.