How to monetize user-submitted links

I have a forum page where users interact and discuss different topics. Lets say one user submits a link to a sales item hosted by an online retailer (ex. macy's sweater on sale on macys.com). How can I, as the site's publisher, monetize on this user-submitted link that one of my users sumbmitted for discussion?

Ask.Metafilter.com automatically adds their referral code to any Amazon link included in any comment a user makes. This works particularly well for them since the nature of the site and its responses lend themselves to book recommendations, and there's only one major retailer of books that people link to online.

For your forum, analyze outgoing links your users post, and see if there are clear leaders in retailers (most often linked to). If so, see if affiliate accounts are available from those retailers, and then add a script to add your affiliate code to any links to their sites. If the retailers your users are linking to don't offer an affiliate plan, they're still good targets to solicit advertising from. Showing them that your users are often linking to them (assuming you have enough of them, and enough monthly traffic that might follow the links to make it worthwhile) already indicates interest in their products.

Ideally, if the products your users are linking to are highly related, and you have the resources to develop custom functionality for your forum, building a 'product search' widget that lives next to the 'Add Post' / 'Add Reply' text entry, can funnel searches for products, and the resulting links, to the sites of retailers you already have an affiliate account with.