Hi guys, I'm VERY new to bitcoin and have an extremely basic question (which I'm still not sure about after all the internet reading I've done).

Let's say a guy on an online forum offers to sell me some bitcoins. He says he'd send them to me, they would appear in my bitcoin wallet and then I'd send him money.

Are there ways this could be abused/ a fraud? Can he, for example, send me fake bitcoin? Or does my wallet verify the coins that arrive? Or is the transaction process such that no unverified transaction can arrive into my wallet? I have some vague idea that the last option might be correct, but I need something more than a vague idea.

Thanks.