The person (s) does have a real fetish. I would think that as long as the person (s) is over 18 it would be legal. I can't imagine that a legislative body would pass a law making the sale of used condoms illegal.
I can see the headlines now!
Not sure what you mean by used undwear. Do you mean selling clothes that have been washed. That is very legal. Or did you mean used as in having cum spots all over the underwear?
Still, can't see that as an illegal activity in a face to face enounter.
I'm sure the health department wouuld shut down a store selling such for sanitary reasons unless such items were sealed tightly in a bag.
These is a bigtime fetish. Monica kept her dress much to Bill's chagrin. Never heard of sellling "used" items before though.