Reselling used software simply is not possible.
Not legally at least.
When you buy a game, you effectively don’t buy the software but the right to use it. That right can only be registered once.
This is simply NOT TRUE, for software "in-general".
As long as the "seller" stops using the software, and uninstalls it completely, the software, and it's licensing CAN be transfered to another user, who then must abide by the software EULA, and re-register the software in their own name.
As to Trainz software, when talking about a physical copy of the software on a CD/DVD disc, it IS 100% legal to sell/transfer said disc to another person, provided the original owner/user follows the above "rules". That said, it is also 100% at N3V's discretion, as to whether or not they allow the old software to be re-registered under a new name. IIRC, N3V basically require the new user to buy a new serial number and "license".
--Effectively, though "legal", there is no good reason to buy Trainz software second hand, as you will need to still pay N3V in-order to use it with any "Auran DLS access" or "online features".
-The only time it is "effective" to buy used software is when said software is obsolete and no longer available from the original "creator" .. Which I believe, ALL versions of Trainz are still available from N3V, via some special edition box set sorta deal.