[Assume that nothing in this post is work-safe.]
A LiveJournalist with a newly acquired post office box (no link in case she has a technically-public but intended-as-private site) says that she'd like to go into the used-panty business. Her fiance says that this should be fine as long as she notes that these are indeed used -- which presumably is the selling point -- so there is no false advertising. However, one commenter claims that selling worn underwear is illegal. This is true in Tokyo, and in the rest of Japan in the absence of a license, but to my knowledge, not in the United States. After all, the sale of all other used clothing items, including some that certain people consider problematic, is legal, and as of January 2001 underwear could be shipped through the U.S. Postal Service.
The real difficulty, in all likelihood, will be finding an online auction site that permits one to sell unwashed secondhand clothing. Both eBay and Yahoo reportedly do not want unclean underclothes to be retailed on their websites (although it undoubtedly happens; looking for women's intimates on eBay pulls both "used" and "worn" as suggested search terms), so the LJer may have to find websites that specialize in adult items. Due to the nature of their merchandise, these may charge higher fees and otherwise be more difficult to use than the most popular auction sites.