Got to protect yourself always!
Check if they own the domain that they are selling links on, by having them place a meta tag to show you they have access to update the site.
<meta name="site-verification" content="L9PWhRxdHhSvaDxtGkG0dHsqpUY" />
or some such thing that you can take a look at the source code of the site and see that he/she does in fact have the ability to update the site
Then use a form of payment that you can get back if they don't honor their side of the agreement.
And always remember, if it sounds too good to be true, then it most likely is
