Google Penguin has a good nose for sniffing out whether the links on a source site are there because they are paid for. If they smell like fish, they’re fishy and Penguin is onto them.
When payment is made for a link, it’s blatantly unnatural. Usual tell-tale signs of payment required on a source website are fairly simple: payment gateways, text about terms and conditions of buying the link, pricing etc are all evidence of paid link schemes.
If you believe that a site offering paid links might benefit your website, then check that all of the tell-tale signs of the financial nature of the link are not visible to Google. That said, Google also has processes to manually detect paid link networks and doesn’t just rely on software to track them down.
I would avoid all paid links entirely as they are seldom worth the investment and risk.