There are several basic steps you'll want to consider, depending on how professional the transaction is, and how much you want to protect yourself financially.
A lot of cats simply will make the beat and sell it to the buyer for a flat one time fee, handing over rights to the buyer to use the beat however they see fit. This arrangement is useful in situations where you have no concerns over how your beat will be used, and don't think you stand to profit from what the buyer will use the beat for. Technically you are also free to resell the beat to other interested parties, if you so choose.
A second scenario is where you would lease the beat for a particular project, or for a certain time period, with the understanding that after the agreed upon time period expires, you will be free to do further business with the beat with other interested parties.
For more professional transactions, you will want to have your material copyrighted (
www.copyright.gov/ ). ($35, I believe, although if you bundle your beats together as one master package you can copyright many at once for the same price). This will give you ownership of the beat and help you in the event that you are owed money at some point based on the buyer's successful usage of your beat.
You will also want to consider joining ACAPS, BMI or SESAC which are companies that, once you register your works with them assist in the collection of royalties and other dues associated with your sold material. In establishing a publishing set up, there are other considerations as well, (DBA or 'doing business as' name and checking account, PO box for transactions, etc.) It can seem complicated at first, but once you get established it's fairly straightforward. You would do well to do some internet searches on the subjects of music publishing, and royalty collection as there is a abundance of this literature online.
These are just the basics. If you want to get up and running right away you might consider the bare bones option listed at the top of the post, to get your name out and drum up a little business, but understand that you have minimal legal protection from someone jacking your material. Of course you would keep your masters but that's basically all you've got.
You might consider setting up a website to showcase some of your material (or snippets of it) to increase your visibility and availability. When someone wants to buy one of your joints long distance you can either send it to him as a stereo two track, or broken down track by track so they can mix the levels independently. You probably won't go through all that if you are exercising the one time fee option.
peace-
LEF
