How much would you pay for copied albums on cassette compared with the originals? Most people here are a little twitchy about copied stuff, as selling it could get you in trouble with copyrights etc.
As for the value of a used Amiga, this will depend a great deal on the model/condition. A500s won't fetch much used as they're pretty obsolete these days, and A1200s don't tend to get very much as there are a lot of them about. Any accessories will help (i.e. original monitors, packaging, scanners etc) and if your system is modified then that will also be of interest - standard Amiga's are very rare these days.....
However, if you've something rare and exotic (A3000/A3500/A4000 etc) for sale then get in touch :-D
Otherwise try
www.amibench.org for a rough guide to used values.