Same reason some people will sell a car they know for a fact has major mechanical issues and not disclose the problems or even fix it just enough so you can't tell but not actually fix the problem. Its partly money and partly I think some people truly enjoy screwing people over, it gives them a sense of satisfaction that they successfully tricked someone.

I do believe there are a lot if people who are honest though. And then there are the ones like mentioned earlier that are just ill informed and trying to sell a product they don't really understand, which is also not good.

You just have to be careful trusting anyone you don't know and if you've heard anything bad about that persons integrity, really really think it through before giving them money.