It depends on the modification for me. If I just swapped out some parts that didn't require any gunsmithing; like grips or a magazine bumper I would put it back to factory and include the parts in the deal.
If it has different sights on it, I'd sell it as is. I don't see the point in pressing out sights when the new owner may prefer them the way I did. I would include the factory sights with the deal though.
I can't answer to extensive modifications because I don't do them. The firearms I purchase are just fine as they come from the factory with the exception of a few minor mods. I wouldn't purchase a firearm that needed a lot of extensive work to perform. I need more work than the gun does, lol.
If it has different sights on it, I'd sell it as is. I don't see the point in pressing out sights when the new owner may prefer them the way I did. I would include the factory sights with the deal though.
I can't answer to extensive modifications because I don't do them. The firearms I purchase are just fine as they come from the factory with the exception of a few minor mods. I wouldn't purchase a firearm that needed a lot of extensive work to perform. I need more work than the gun does, lol.