BINGO! I don't promote my link sales based on PR, I focus on selling my links based on my traffic patterns and the nature of my content. From time to time advertisers will buy links to non-relevant sites, but for the most part the advertisers that really stick around year after year are very relevant and are targeting my traffic as much as much as anything else. Besides if selling a few irrelevant links helps provide extra income that allows me to buy more articles than those links still help add value to my site, just indirectly.
If you are asking because you are interested in what factors influence credibility as far as users in general are concerned then this is a good place to start - Stanford Guidelines for Web Credibility I've just written about Page Strength and PR. - Michael
Nice article Michael - probably a bit too in depth for the average webmasters buying and selling links, but it would be good to see people looking at something other than PR.