On a realistic level, in terms of worth judged by the links possible effectiveness for helping someone rank, it depends on the site's backlink profile. Some PR 5's are fake or meaningless. I've seen some sites whose profiles consist of comment spam. Some of the comments are even on Matt Cutt's blog (how dumb is that?). Another determinant of value is what the site is linking out to. PR is not the only metric.
On a less realistic level based solely on PR, I believe there are enough link buyers focused on PR who will pay you amounts based on that metric. Your niche is competitive so there should be a healthy market for your links.
Regarding a marketplace for your links, publicly advertising that your site is selling text links may not be a good idea. It invites scrutiny and can lead to a competitor reporting your site to the search engines and causing it to become penalized.
Selling links goes against Google's policies because link selling subverts their algorithm. Publicly offering to sell links, whether from your Advertising Page or posting it in an SEO forum is not sensible.