Well, I've mentioned this before. The features of a store are important, of course. But what's even more important is the people behind the features. When you invest in any business, you're really investing in the people who make up the business, and I really believe Soundgine is working the hardest to please their users and adjust according to their feedback.
And I've been on everything... that said, just so you know I'm coming from a neutral standpoint.. myflashstore did just release a whole new update for their site which is looking pretty nice. I think myflashstore is good, but I think soundgine is better.
Definitely places like rocbattle, soundclick, and beat stars all suck though, for real. I've made lots of random sales on rocbattle, but they control your prices and can change them when they see fit, and they charge you $50 to have any beats removed from the site. Beat stars is cool because they occasionally have neat opportunities similar to how reverbnation has opportunities primarily for bands/rappers/singers/DJs. But pricing/terms is sketchy also, and on both rocbattle and beat stars, you cannot refer people to an outside site, because they are afraid of losing traffic (because they know their site sucks and people would leave in a heartbeat lol). You cannot even show your email or anything. I've had people make offers on my beats, and I didn't see the notifications until it was too late. Beat stars didn't even email me a notification alert or anything.
As far as soundclick goes... it... doesn't really do anything... it's not made for selling beats AT ALL... the only reason it's popular is because it's popular. But it's been shrinking for a long time, and more and more people are catching on and realizing this. They are just as sketchy as rocbattle and beatstars, maybe even moreso?... well, I don't think you can top rocbattle for sketchiness... but the soundclick owners/admins don't give a crap about beat makers, yet it's 99% of their traffic. Soundclick has been caught using sketchy fake traffic tactics and a lot of my soundclick buddies have been going on about it more and more. Even Superstar O is getting fed up with it, which I'm pretty sure is why he jumped onto soundcloud within the last year.
Nowadays, I believe selling music is all about establishing a presence on a social network, i.e. facebook, youtube, soundcloud, twitter, instagram, etc... and then getting either soundgine or myflashstore to make the sale. And like I already mentioned, soundgine is the better in my opinion. I've had myflashstore for a long time, and I have no hate for it, I think it's great... I just think Soundgine is better.