Each outbound link on reddit is tracked, similar to how Google tracks users clicking on SERPs. This can be tied back to a user to build profile on your interests, in addition to sub-reddits you subscribe to etc. which can be valuable if combined with other demographic data about you.

I dont know if they started selling this data directly. Last I spoke to ad people there, they had plans to build out their own tracking and analytics infrastructure, to improve things like geo-targeted ads, as their capabilities were lacking signficiantly.

You can turn off outbound link tracking in the privacy section of preferences. Uncheck “allow reddit to log my outbound clicks for personalization”

which means your logged in and your behavior and activity is captured, and available for ad-targeting anyway. There's no real opt-out.