Yes. Just give them a wash as usual and they’ll be fine. Anyone using second hand clothes will likely wash them how they usually would.

Of course it is. So long as they are in good used condition and washed I don’t see a problem!

Yes; just make sure they’re washed first (or as soon as you get them, if you’re the buyer).

I bought and sold all of baby clothes and mine as well.
Just wash them well and use wash sanitizer with first wash.

I would say generally yes it is safe. I wouldn’t sell underwear but if it’s just other clothing items I would be happy to buy it and give it good hot wash to sanitise to be safe.