In the past, I've done mixes for a few folks - Critter, Curt Young, Joanne.
Not something that I would care to make a habit of - I am almost ALWAYS working on music - writing or producing - so doing it for someone else "gets in the way" of that. Mixing someone else's music takes more time than doing your own, I think...and can be time-consuming.
I "mastered" something for Lawrence (gruverider) - which was little more than running one of his mixes through some Ozone presets in order to convince him he should buy it (which he did).
I've done productions for folks in other music forums using BIAB tracks - or generated some BIAB tracks to add to their songs - in order to convince them they should buy BIAB...Most of them have - how could you not??!!??

I think it is a good idea to send your mixes to someone else to listen to before you "release" it. Someone whose ears you trust to offer any "tweaks" that might be needed. Janice & Bud hear most of my stuff before it hits the fan. I've turned to Tom Adams a couple of times for a listen. Ears I trust.

150+ Songs produced using BIAB RealTracks
And 50 more...
and some more...