Okay, sure, lets' get it out of the way: EWWWWWW!
Now, then, this young lady is part of an industry that transfers smelly socks, dirty underwear, and even more icky things to people (men, likely, but who knows?) who ... use these items in ways that these buyers value.
Is the exchange euvoluntary? The young ladies can get $50 for a smelly pair of socks, and lots more for a pair of worn underwear along with themselves wearing the underwear.
Now, on the scale of porn filth flarn filth, a picture of a 20 year old woman wearing modest underwear is pretty tame. Those pictures are in the Sunday adverts, for heaven's sake, in the newspaper.
Of course, one could argue that only a crazy person would pay that much for soiled undergarments. But no one is being harmed. Or are they?