I've been wondering the same thing. I've been using SoundCloud for the longest, but I never really thought it was a place rappers went searching for beats, more of a place to search for mixes, etc. etc. A "showcase" like Isk2009 said. I have a SoundClick, but don't really use it since I've read some bad stories on here about people getting ****ed over for "copyright infringement" and whatnot. ReverbNation looks more like a place for artists such as rappers, bands, etc. to sell their music/albums on than for a producer to sell beats, same with Band Camp.
I'd love to hear some suggestions, as right now, even though I've only really sold 1 of my beats, I'm really starting to want to try and promote my music and what not, but am mainly relying on my Twitter posts, forum signature links, and just spreading my contact info throughout websites I usually visit.