Afaik, negotiations can only be done at the PC.

I've found it very hit and miss, either the other party want far too much ... or they never accept/reject, leaving you in a position where you either A) Leave your tokens tied up with that players offer or B) Potentially missing out on a good signing.

Personally, if I see a PT'd player then i'll offer. I'll only offer once though, wasting your time and tokens otherwise.