It depends on the reasons for the text link. Does your site have a good Pagerank? Google has made it very clear (even modifying the Webmaster Guidelines recently) that paid text links for purposes of traffic are okay, but if they are an attempt to gain Pagerank, the buyer and/or seller likely will be penalized for it, both in Pagerank and in the rankings.
The way you avoid this is to add the rel=nofollow tag to the link. If your prospective advertiser objects to this, then he probably wants to suck off your Pagerank, and I'd avoid it.
If he has no problem with it, then go for it.