Buy box on retainer is how private lenders describe a wholesaling arrangement that does not fit the usual model.
A private lender describing how his money moves will sometimes mention that two of his borrowers pay a wholesaler a monthly retainer to hunt for them instead of waiting for blast emails. He calls it buy box on retainer and moves on as if everyone knows what that means. The standard understanding of wholesaling is that you put a house under contract cheap, then sell the contract to an investor and keep the difference. That is the spread. In the arrangement he described, the investor is paying the wholesaler money every month whether or not anything closes, and it is not clear who ends up holding the contract on the house. Is the wholesaler still the buyer on paper and then assigning, or is he just handing over addresses and letting the client sign? There is also a claim on other sites that if you get paid for finding a property you never had under contract, you are a bird dog and that is illegal. That cannot be right in every state or nobody would advertise this.