Why not just have a sign up NPC near the wvw portals in lions arch? In order the participate in wvw you must speak to this NPC and sign yourself up for that server. If you transfer you leave the previous servers roster until you sign yourself up for the server you transfered to. I would also say that after a certain time period you get removed from the roster after you don’t participate in wvw after a certain time period. This way you get a somewhat accurate count of wvw players on a server and can base transfer fee’s on that rather than the overall server population.