They don't mind however they would prefer you to use the no follow tag when selling links, ie
<a href="http://www.example.com/" rel="nofollow">This is a paid link</a>
That way the person buying links does not benefit from pagerank flow or from a vote from your site to it, they see link buying/selling as altering their organic results and have penalised link buyers and link sellers who set out to sell pagerank/ranking links.