I'm on Redbubble! I ordered a few of their prints to make sure the quality was good (attempts at photos) and I tried out sticking something on one of the new graphic tees (another attempted photo)! I felt good about both! I can't speak to Society 6 'cause I haven't tried that one yet, though I have seen people on both.
I feel like I heard somewhere that INPRNT has a higher margin for artists, but they're moderated so you have to apply to get in.
My thing is, I would make more profit selling prints myself, but then I have to keep a stock of posters, I have to drive out to the post office and actually send them, and there's risk involved -- if I get 20 prints made and I can't sell them, then I've lost money. If I tried to get prints made one at a time on demand, then they'd be a lot more expensive than to make.
And that's not even mentioning the other products that would be very hard for me to ever make myself!
So for me, it seemed worth it to switch to letting Redbubble handle things! I haven't put a whole lot of effort into merchandising yet, but it's literally risk free. I can put any silly thing I want on a shirt and just see if anyone wants one, and if nobody does, there's no loss. The way I look at it, that's what you're paying for with these services -- complete lack of risk.