Do you get penalized for selling links?

[Molat]

What is google's ruling on selling links? And how do they know which links are paid and which are, for example, link exchanges?

For example, I run a football blog and have just sold one text link to an online bookmaker. Will I be marked down by google for that?

Does the nofollow attribute play a part? And do different rules apply to wordpress blogs? Is the nofollow attribute set by default on WP?

Any information is welcome.

Thanks.