You can't sell exclusivity on a public record, so stop trying. What you can sell is first delivery plus a time-limited hold, and that's what the agreement should say.
Structure it three ways. First, delivery log with timestamps, as trellis said, and the log is the sole evidence of who got what first. Second, a hold window: buyer A has 30 days from delivery to put the parcel under contract or make documented contact with the heirs, after which the lead goes stale and you're free to deliver it to B. Thirty days is arbitrary, pick something matched to how long heir property actually takes in your states, which for scattered heirs and a possible probate is usually longer than a normal pre-foreclosure. Third, an independent-source carve-out. If B can show a dated record predating your delivery, no fee is owed, and the burden of showing it sits with B.
At 750 flat on close, your economics don't support a fight over any single parcel, so the structure exists to make the fight not happen rather than to win it. Consider whether flat fee is even right for land with back taxes. A parcel where the tax lien is 4k against a 35k value has a wildly different outcome for the buyer than one where the taxes are current, and you're paid the same for finding both.
The exposure you haven't raised is heir property specifically. Scattered heirs across three states can mean fractional interests, unopened estates, and a title problem that kills the deal months in, after everyone has spent money. If your fee is on close, you get nothing, and the buyer remembers that your leads don't close. Screen for whether an estate was ever opened before you deliver, and say in the packet that you haven't verified title. How the heir interests actually resolve is state-specific probate law, so that part is an attorney's call, not yours.
