Heaven forbid you should have to delete a message. It is so much work.
Banned for a week? That's too lenient.
I say we ... Pee-wee's Big Adventure - I say we kill him! - YouTube
Got a PM from a noobie? Just mouse over to the delete button and click. It takes a few seconds, what's so hard about that?
I understand there is common sense that dictates one shouldn't PM a seller about a closed thread, but there's common sense and then there's how things are often really done. Yes, you shouldn't PM someone whose item is sold, but many people do and there is no real way mods could give out infractions for every person who does. The only real way to fix this issue is to learn how to be a patient TB seller. If you take the advice I've given then you won't stress out about a few silly PMs and you'll find your experience in the TB classifieds to much more rewarding than the alternative (CL, Ebay, etc...).
There are how things are supposed to work and then there's how they actually work. You can stomp your feet all you want about it but it's not likely to change anytime soon.