Buyer with a defined buy box wants an exclusivity arrangement. How do people structure that without turning into an unlicensed agent?

The flat fee and the licensing question are separate problems and the second one is the expensive one, so take it first. Whether an arrangement crosses into brokerage depends on your state's statute and on how a regulator there reads it, and that's a question for a real estate attorney licensed where you're operating, not a forum. The general mechanism that keeps assignment on the right side of the line is that you hold an equitable interest in the property through your own signed purchase contract and you're selling that interest. If you're being paid to find and negotiate property for someone else without ever contracting to buy it yourself, the analysis changes, and several states treat that differently again. Exclusivity plus a standing fee starts reading like compensation for a service rather than profit on a contract, and that's exactly the framing you don't want in writing. Get the agreement drafted so the fee is consideration for assignment of a specific contract, one per deal.

On the economics: a $6,000 flat is only bad if your spread distribution is wide. Two a month is 24 deals a year, which at $6,000 is real. Your $18,000 deals exist, but so do the ones where you'd have fought to get $4,000 and eaten a cancelled due diligence period. Pull your last twenty comparable sourcing attempts and look at the actual distribution before you argue for a percentage.

What you haven't raised is concentration. One buyer at two a month is your entire business, and if their capital source pauses or they change markets, you have no other relationships and no list. Negotiate exclusivity with a term limit, six months, and a minimum purchase count that voids it if they don't perform. Otherwise you've given up your other buyers for a promise.