I don't think that Googlebot has learned to read text like "Buy a totally undetectable PR8 link now for as low as 2,000$ and get four PR6 links for free!" on images. Sites operating that obvious link selling businesses leave other --detectable-- footprints, and their operators are aware of the risks involved. Buyers are rather save, they just waste a lot of money because purchased links most probably don't carry link love and nofollow'ed links may be way cheaper.
I often stumble upon cases where old and forgotten links create issues over time. Things that have worked perfectly in the past can bury a site on todays SERPs. The crucial message is be careful who you link out to! Compile a list of all your outbound links and check them carefully before Google's newest link analysis goes life. Especially look at the ancient stuff.
Update: I've just spotted Eric Wards article on forensic inbound link analysis, you should read that and his comments on outgoing links too.
Tags: Search Engine Optimization (SEO) Paid links are risky with Google
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