Hiring for a link builder can be a unique challenge because you need to figure out the unique skills set needed for this particular role - pure SEO knowledge just won't cut it.
Eric Ward has a good article on SEL about what to look for in a link builder, A Portrait of the Perfect Link Builder.
Interestingly, SEO isn't really what he's talking about. Eric describes all the soft skills required to be successful at link building.
While you may be searching for the ultimate candidate that has all of the listed characteristics, plus an SEO background, my recommendation is to look for SEO aptitude, but focus on the soft skills and characteristics that Eric describes. The soft skills, passion and the other characteristics identified are far more difficult to teach than SEO.