It depends. The site might just be putting links just to share it with it's users/visitors...However, there is a chance that these companies can pay to advertiser their website on your website...this usually happens if your website gets tremendous traffic. Now it's upon you and the buyer to choose from Pay Per Click (PhpAdsNew is a good free software to keep track of clicks and impressions) or just advertising where you get a straight amount per/month.
I am not sure of any program which will arrange it for you. However, if you have the right keywords, then just join Google Ads and they will show ad relevant to your keywords...It pays off
