While some SEO's will be in pursuit of the PageRank value that can be passed to their domain, the best value you will get from buying links is in the traffic they provide.
Age of Domain: While the quality of the site should clearly play a role in your decision to purchase a link, the age of the domain (and not how long since it was initially registered) is an excellent starting point to determine the value a link could provide to your site. While there are many Web services available which show you when the site was initially registered, the best to determine the history of a website is archive.org. Not only will you be able to determine if and when the site has changed hands, but whether the focus of the site has changed.
Placement: Determining where the link will actually be located should play a major role in your decision as well. Is it a sitewide link (good) or on one page (not so good). Is it within the content of an established, indexed article or in a sidebar (much less visible). The only way to determine where you site will be linked from and in what manner is to ask the link seller. The relationship you have with this group or individual will go a long way towards ensuring your link is treated the way it deserves.
Rank & Relevance: There is clearly no special formula, no silver bullet or magic bean to buying links. However, the rank of the site for the target search term at Google, Yahoo, MSN's Live, etc. is an excellent starting point. While many prospective link buyers are hesitant to purchase links from those appearing higher on the SERPs than they do, modifying your intial query and selecting a site which compliments your own and does not compete (but still provides value in terms of the potential of its end-users to visit your site) is the strategy those most familiar and fundamentally vested in link buying follow. This by all accounts is relevance.
Backlinks: In the end, it comes down to backlinks - always has, always will. Sites with more backlinks are better for purchasing link from than those with fewer. There are so many tricks to artificially boost your link counts so that it appears the link count is high when it is really not however, that link buyers need to know how to determine the quality of the backlinks pointing to the site from which they are considering purchasing links. For example, using Yahoo! link buyers can enter linkdomain:Seller.com -site:Seller.com to figure out how many genuine inbound links are indexed by that search engines. You can take that a bit further and remove other sites as well, for example linkdomain:Seller.com -site:anothersite.com -site:anothersite.com.
So should you buy links to aid in your search engine optimizatin campaign? My thought has always been Yes, absolutely yes - but there are some caveats. If you think someone can tell that the links you are receiving (or would be receiving) have been bought (and I just essentially showed you how) then it's time to rethink the process. Much like traditional link building campaigns, link buying takes time and a careful and cautious eye. In the end, buying links from relevant places that provide traffic, and not Pagerank, is the best course of action.