3DSM retails for around $3K, with educational versions going in the $1K range.
With a "used" copy I wouldn't expect to get even $1K for it, and e-bay more then likely will shut you down if you put it up(happens all the time with 3DSM, Lightwave and many Maya versions).
I wouldn't be surprised if you could get $600-$800, possibly even more though that is what I would expect you to pull in for the most current version(which one do you have?).
It isn't a "simple" program, but nothing with 3D is. After you learn it it is an exceptional piece of software(though it takes time, I haven't spent too much time with it myself, but it is very nice from what I have seen).