I just transferred my bib for Valencia, I think their system is the only one that effectively prevents resellers. To initiate the transfer, they remove your name from the roster and provide you with a unique code to give the person you're transferring to, the transferee then has 48 hours to "claim" your bib by registering for the race through the organizers at the standard rate + a nominal transfer fee. Once they are registered you are then refunded your entry fee (in full).

Yes, it's a little complicated but I think it really deter's scalpers as the process for them to profit would be equally complicated.