This probably won't make friends, and I'm probably not 'teaching' anyone anything... but..

it isn't 'good karma' (never mind legalities) to make mixes (i.e. DJ sets of other peoples music) and sell them without permission/licensing. Can you get away with it? probably... Does it really cause damage? maybe, maybe not. But even if not-for-profit you should get permission, else keep it in closed circles of friends. There should always be a tracklisting, so the artists get a touch of promotion/credit at the very least, maybe a sale or two if they're lucky.

I'm shocked by how many DJ's don't include tracklistings, to me it is as if they're trying to pawn off other peoples' music as their own.

(e.g. "DJ X Hype Set #423")..
Then same DJ emails: "Check out my mixset with your tunes!!!!"..... like I've never heard my own music before and I'm supposed to be impressed.. "wow, he put a flanger on the outro of my track....how did he ever do that?!?!".
what ever happened to asking permission first, or at least crediting the artists??

..with that said!
for selling original music:
-Beatport.com is great for dance music, they don't let just anyone on however (you need to be a formal label and meet quality standards). Its been very good to us, a ton of DJ's are using Beatport these days...
-CDBaby is a time-saver if you want to get on all the other major download sites (they take a cut, but it saves you a lot of grunt work).