Is the TiVo registered to your account on tivo.com? If not, do so.
It's a dilemma. The buyers want the TSN in order to verify that the box indeed has lifetime service. Maybe they hope to call TiVo and ask, though the results are not always reliable.
Unfortunately, it IS theoretically possible in certain circumstances for someone to transfer the lifetime sub to another box. Very unlikely if you have it registered to you. What often does happen, and this is why they ask, is that the seller sells the box and THEN transfers the lifetime status away before it is reregistered.
I'm not sure what to advise you here. I think that as long as you have the box registered, it's reasonably safe to give prospective buyers the TSN.