
Private message confirmation system (if possible)
Well, with dynamic IP's I do believe that vBulletin keeps a decent log of IP's used by a member (maybe with plugin or native support) and most dynamic IP's will at LEAST have the same first 2 sets of octets They are called "Sticky Dynamic Ip Addresses"
For instance: Person A has a current ip of 120.345.23.1 and reboots router or computer and his DHCP lease renews with (in MOST cases) an Ip of this sort: 120.345.xxx.xxx where xxx is the dynamic range of the IP. As most ISP have a lease on a specific set of IP's it IS possible to at least know that a member of a vBulletin forum has the same ISP.
Now, given that it isn't 100% fool-proof, it could be implemented in the PM by it saying something like "This user has a Dynamic IP address and has been using the same ISP for the past XX days"
Just a theory or a solution to the problem. May post further ideas on the matter.
To the OP, I support this as it simplifies the trading process.