How do I prep my rooted and moded phone for resale?
If I was buying a used device, I would expect that a factory reset would have been done to it. Either way, I'd probably do a factory reset myself anyway, just because.

Unless, of course, I rather wanted a rooted phone.

I think a factory reset is the better option. You wouldn't want someone to stumble upon some of your personal data.

In any event, it's best to make sure the buyer knows exactly what they're getting.