I don't know about using eBay as things are all over the place. I have paid from $0.99
+ s&h to $29.99 w/ free shipping (for something that is useful e.g. HMS for 5150 in
unopened condition). My average seems to be $9.99 for more popular stuff and $4.99 for
esoteric stuff (say GTO to the 7327 plotter). Every once in a while things sell for crazy
prices (do you really need the TR to the cluster network for $90?). What sells high seems
to be SW specially DOS 1.0 and IBM branded CP/M. Those are fetching >$500 these days.
Just my two cents....
-Ali