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.