The main thing is - charity generally doesn't get people to buy things they have no interest in or can really easily get for free elsewhere - what charity does is generally to lower peoples demands regarding whatever is for sale through charity... the fact that it's for charity makes it much more likely people will buy the item provided it's something they would like to own anyway... even just a little bit.
Anyway, to sum it up... this is how it works in my experience -
Charity makes people much more likely to buy items as long as the items in question are of interest, even if the interest is quite mild.
Charity does not make people all that much more likely to buy things they have no interest in purchasing whatsoever.
So the problem is finding something you can sell that people would find interesting... I'm sorry I can't help out with that but I'm thinking your idea of a CD32 disk containing full images of lots of shareware games sounds like quite the good idea - provided it's possible to do. I have no idea how it is with Amiga shareware and how much work it is to track down the various creators and getting their permissions... it does sound like quite the project in any case!