The larger parking companies like Sedo, Parked, Afternic all have accounts at multiple registrars. Domains can't be transferred to a different registrar, but they can be pushed to an account with the same registrar.
For example I registered a name at GoDaddy in January, parked on Sedo, got an offer in 3 days, and sold it at auction a week later. I push the name to their account, they push the name to the buyer's. This means if the buyer didn't have an account at GoDaddy, they would have to have created one, but it's free right?
P.S. Don't expect to get offers that fast, it's only happened to me once.
