When your product drives a real-world outcome you cannot directly instrument — Apartment.com and Rent.com both faced this with the signed lease, an offline event with no API into a landlord's paperwork — there are two workable strategies.
Apartment.com's approach: trust-based pricing. Charge a subscription and don't try to prove attribution; let advertisers assume the traffic converts, and price the product on trust and reputation rather than measured ROI.
Rent.com's approach: cash-incentivized self-reporting. Pay renters directly (a rebate/incentive) for confirming they signed a lease found through the site, turning the user into the instrumentation.
Neither is objectively better — the choice depends on market trust dynamics, whether your buyer will tolerate paying without proof, and whether you can afford to pay users for data. Casey used this as a general model for any product with an unmeasurable core conversion event, not just real-estate classifieds. Related: A Second Product Only Needs to Fix One Weak Link.