In regards to sales on these sites. I did a couple of test runs on audiojungle and pond5 with sound effects and a couple of tracks (music is not really my strongest suit though). I basically let it sit there with no extra advertising (i.e. social media etc.). This generated zero sales over the course of 3-4 month.
I came to the conclusion that in order to make this work you really have to prepare a ton of content (maybe >100 tracks) and also maybe do some sort of advertising to push people to your stuff. This takes a lot of time away from the time I should probably better use to push my freelancing. So I dropped that idea.
If I every have enough tracks ready I would first try to get a licensing deal with one of the big players like
www.bmg.com. Their clients are TV/Radio channels.
However, I've also a couple of small sample packs on the Unity3D Asset store. And these generate a couple of sales on a monthly basis. I'm not sure how well music does on there, but I assume devs are more drawn to the free stuff there as well. I'm definitely going to explore this more in the future when I have some time to kill.
Regarding running your own store. I think it's all about convenience and whether you want to handle all the extra work this involves.