A legit purchased text link passes relevant traffic, but it doesn't sway the search engine results. Google doesn't care if you buy advertising, they just don't like it if you do it to try to manipulate your position in their SERPs. If the link uses javascript or is nofollowed, there won't be a problem.
Nobody says "you can't do that." But do paid links pass any real PR value anymore, or have they been devalued? How do you know the PR of the page the link would be on? The toolbar is months out of date, so if a page was touted as an 8 it could be 0 next week. Is the seller showing you a PR8 but redirecting you to a different page that's a PR0? Do buyers even check? Will there come a point when Google updates its little green toolbar less and less often?
Links in the content of relevant pages for the purpose of traffic will be fine. Links on pages titled "Links Page" next to a list of non-relevant links are not going to boost PR. It isn't hard for a search engine to tell the difference. Nobody says you can't chase PR, either. It's just that PR won't be worth as much as trust rank.
Here's where Matt talked about link devaluation:
http://www.mattcutts.com/blog/google-hell/
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