I'm on MyFlashStore and I make a nice and steady income there from selling beats. However, the marketplace of MyFlashStore doesn't really work for me. So if you're using MyFlashStore, make sure you integrate the shop/player on your own website and drive a lot of traffic to your website. However, I've also been experimenting with BeatStars lately, and it's absolutely amazing! I joined a couple of weeks ago, I uploaded about 40 of my old beats and already made around $119 from selling non-exclusive beats on there. So if you're starting out, I would recommend that you roll with BeatStars. It's still new and you can get a lot of exposure. SoundClick used to be great but now it's dominated by the big players and they are messing things up. They deleted all the beats of my friend Anno Domini's co-producer 2Deep after 7 years, without giving notice and for no reason.
I think the SoundClick era is over soon... they're starting to mess up big time.
Another tip: Don't sell your beats under $10 dollars. I'd go for something between $15 and $30 dollars. If you sell your beats at a too low price, people will think it's garbage just because of the price tag. Look at people like Anno Domini, Allrounda, LifeAndDeath who are all selling beats at 29.90$. I sell my beats for $19.90 and have a lot of success with it, but I'm planning to rise the price soon...