Sometimes said rifles come with a bunch of accessories, or some people have had custom jobs done (fluting, trigger jobs, barrel being cut down, etc).
So sometimes the goodies make the rifle more exciting and worth a little more, since so much has been put into it. My rifle has a fluted barrel, a phantom flash hider, a PRS stock, an ERGO grip, the fancy new interlocking Magpul rail covers, a Millet DMS-1 scope, a custom paint job and a vertical grip. I wouldn't even consider selling it for just $1500. Sure, it's used - but it's worth more because of all the good stuff.
My point is... sometimes it is a worthwhile deal because of the configuration. If it's bare bones, no goodies attached, and they want $2k for it... that is indeed excessive.