IME, it's better to start low than high. I generally start adult clothing around $7.99, unless it's NWT or something uber-special. Plus-size clothing generally tends to do pretty well.
Do a search of Lane Bryant in ended auctions to get a feel for what sells, what keywords to use in your title, and what shipping generally runs in successful auctions. You may do better listing the Old Navy, Sears, and JCP together in a lot, but doublecheck in ended auctions to see how well they sell seperately.