I would like to suggest that you allow members who received a temp ban should be allowed to keep their pm privileges or at the very least be allowed to receive and answer pm:s. The reason for this is the marketplace and that it is a pain for other members when someone you do business with already gets a temp ban for an infraction and you can't continue to communicate with them through the channel that had been used to that point.
IMO if you are going to be doing business on DP, you should makes sure you don't do anything to get a temporary ban. But if you are on the other end of business, make sure you get contact information right from the start so there will be no issues if they get banned.
I agree completly as far as that the sellers shouldn't get temp bans (usually for bumping threads) but I still think it's a shame that the buyers should have to do more work because of it as it happens to very established sellers.
That can be a problem which is why I added "or at the very least be allowed to receive and answer pm:s" It is a bit hard to spam if you only can reply to pm:s. I would support making spamming a permanent ban. However it might be so that this would create more problems than it solves.
PMing is a privilege, not a right. It's for members who follow the rules, which is not hard to do. I'd imagine people would no longer have an urge to do business with banned members. Users who have been banned temporarily usually get banned for infractions. You need 10 infractions to get banned, which means you broke the rules multiple times.
If you're doing business with someone get their e-mail or alternate form of contact. Relying soley on PM's for communication is a poor business practice. Banned is banned, allowing banned members (temp or otherwise) access to to the PM system isn't going to happen.
Allowing PM would just lead to some hidden scammers (not saying banned members are scammers). But yes, like Katy said, more moderating, more work. Plus, a punishment has to get through the person eh?