IntellectualPropertyforsale.com is kind of long.
I would try to shorten it up with something like
ipforsale, patentsforsale, or trademarksforsale, or TMforsale.
I've already run an availability search and the one that seems most valuable to me is patentsforsale (patentsforsale.us and patentsforsale.info are still available). It looks like there's no TM problem either.
Just my opinion