Hi there and welcome to the forums.
According to the Autodesk End User License Agreement, selling/purchasing Autodesk software directly to and from another user needs to pass through a Transfer of License process which requires approval by our sales and legal teams.
You can access the Transfer of License form and its respective FAQ on this link.
Best regards,
Autodesk is the only body to determine if the license is legit. That's how it work.
As far as I know, the process is simple: you install LT on the PC with an internet connection. When it's completed, you start LT and it asks you to register and authorize. Do so and it will log into Autodesk and get the authorization codes for you.
If you are unable to do it online as I described, you will need to do so through this website, read all the isntructions.
https://registeronce.autodesk.com/prodreg/beginRegistration.action
Reread the first sentence I wrote in this reply. Good luck.
No one here is going to send you any codes. No one.