Hey,
That is a good question as many others also ask this and never find the answer.
While there are some sites online for selling beats (Google them), I personally don't think this is the best option.
If you aren't already in the position of selling your music/beats, then it would be best to offer that service to people for free and build up a reputation and library.
A lot of beat selling websites promise that you will make X amount of money, but really you just register, upload the music and wait for nothing. Networking is worth so much more in the music industry. Build yourself a name and portfolio.
I'm sorry if this isn't the answer you were looking for, but it is my personal opinion.
MasterSplintar