I know people who just bitch without a solution are assholes.. so I'll try here.
Could you lump new users (<10 posts) into that BS 2 minutes between PMs crew? When trying to sell something, and 5 people respond, it sucks wasting 10 minutes trying to reply.
I understand its to deterr another mass PM spamming, so that's why a noob-class would eliminate that issue, as well as not penalize long-time members.
I know it wouldn't be hard. On a forum I moderate, we've added 4 new classes of users to make life easy on us.
-Jeff