First of all if a perma banned user has a set that I would like to buy, how would I be able to contact him? Would a gm tell me his alt if he has one?

Also so you know who owns this set right now? http://forum.toribash.com/showthread.php?t=490812

Check the contact details on their profile of the banned person, they might have skype/email/steam listed.

If not, yeah you could ask an admin but it's really case by case.

First, that depends, if he has an alt in the community, then maybe you can ask a GM, or alternatively, you can ask people like former clan mates or friends if he has other contact options like Skype, or facebook.
I think those 2 are the only choices you got, if he's gone from the community with no trace, then I think the set dies with him.

Second, there's a thread for that
http://forum.toribash.com/showthread.php?t=449424

Thanks