in the UK you can sell them on
Adult Work and it seems there is actually an active trade from cam and escort girls.
In the US many cam and phone girls do it - depending on whose opinion you get (even from lawyers) there are varied legal opinions. Most of the illegalities stem around US Postal Service regulations and most of the horror stories center around a case in the Carolinas where a girl was prosecuted. She was originally accused of fraud and the authorities looked for anything they could charge her with - what got her in trouble (block your eyes men the next statement gets icky) was the mailing of panties with copious menstrual fluids.
I think the better business to run is a remailing center/billing center/store for many girls to offer their panties - lots of girls want to sell them, just most are afraid of some perv getting their address.