Most SEOs caution against dealing with paid links. As said above, I don’t think you would be penalized for buying them, but some in the industry think you will be one day. Kinda goes against Google’s public comments about other sites not being able to hurt you, but who knows with Google.
As for selling textlinks, I don’t see how using a broker would be any different than selling your links to another webmaster on this site. (Hint, with a 50% commission split, you would probably be better off just selling them here). You can be hurt if the site you decide to link to is banned by Google, so definitely check out the site before agreeing to link to it.
The visual PR doesn’t matter, real PR is in the background, and PR doesn’t much matter at all these days. As long as the page is cached in Google, you get some sort of credit for the link, yes, higher PR will likely be better, but not neccessarily (eg: A single link from a content rich .gov domain talking about online gambling with a PR0 is definitely better than being 1 of 100 links on a PR6 site about home loans).