Hi Dude
Welcome to NP, hope you njoy ur stay and all the best with ur domain ventures.
If you need to transfer a domain after u have sold it.
It would be best to get the seller to open and account with the where ever the domain is currently registered as well, if they don't already have one. Then you can simply push the domain to their account. If they want u to transfer the domain to a different registra then you need to check what the process is with your registrar and ensure that the domain can in fact be transferred out (depends on domain age and TLD)