I definitely understand both sides of the coin, but I personally prefer to sell bikes certified whenever possible. Mainly to expand the pool of potential buyers as much as I can. I think there is an argument you could make that buyers looking for their first bike might be intimidated by the process of buying a bike, getting it safetied, re-registering it as fit, etc...
Of course, when buying bikes I do not hesitate to consider bikes that are not safetied. I am pretty comfortable with assessing a bike's condition and estimating what it might need. And I do like being able to choose my replacement tires & parts.
Edit: I didn't complete that thought - I didn't meant for that to come off as "I am so great" lol, I meant that as a counterpoint to selling it safetied. The type of buyer that is unconcerned about it definitely exists. And there are definitely many more experienced/capable DIY mechanics around than I