Sounds like it'd be worth it to have a service where you can subscribe by address (/"lot description") and get notified if land goes on sale / is being sold. Sign up for your lots to make sure no scam artist sells them. It could pull info from the various MLSes, but to catch direct sales it'd have to also interface with... county treasurers? Escrow companies? "Recorder of deeds" on state level? Coverage is going to be a challenge.

Offer it as a "warning system only", to keep it cheaper than title insurance services. Target land owners, not buyers & sellers. Maybe sell B2B API access to title insurance companies.