So Steam can actually ban an account for impersonating someone and scamming people?

Probably, if the victim provides enough proof.

steamrep is your friend. And steam support might help you too, althought it can take some time.

I bet the legal description in the Steam TOS says that they can ban your for any reason and not have to tell you, or something to that effect. Whether they actually will ban someone or not is a fickle thing.

Is he really impersonating you or is it just an unimaginative nickname?

They can, if they will is another story

I sure hope they do ban them.

Well, write to them ASAP. Steam doesn't really care about scammers - "there are trading with 2 'yes, I want to trade' buttons, and we take no responsibility about any outside steam-trade-system deals", but one mod might take care, but he might not read all right and shit will fall on you.