So, the idea is to sell a prisoner to another realm only if executing such noble is already an option in the realm that captured it. Right?
The restriction I deem most appropriate would be the inability to transfer a noble to a realm where he was banned if you have not banned him yourself.
If haven't banned him, and you ship him to a realm where he wasn't banned from either, sending him there does not increase his threat of death. As such, I don't see any issue with this possibility.