Obviously I can't stop you PMing each other
Ideally things like this should take place in the For Sale forum, either by one party posting a For Sale thread or the other posting a Wanted thread*. Using the proper channel means a) you've got an open and transparent agreement on the sale, and b) you're not encouraging people who don't have access to FS to think they can buy and sell in open forum.
*n.b. Wanted threads have less stringent requirements than FS threads so are probably the easiest way to do this...