Just walk away from the deal and find a different pistol to buy. I second what Tom Servo wrote in post #2.
Several years ago I was selling an older Jeep with a plow rig on it. I had a buyer run me all over the planet. First I took it to his mechanic to be checked out. The mechanic said it needed a new gas tank. (It needed a 5-minute repair of a short piece of fuel line.) Then I had to take it to the distributor for that brand of plow to be checked out. They gave it a clean bill of health. After all that, he offered me about 60 percent of my asking price, which I refused outright. Someone else bought it on the spot for the full price. A week later idiot-child called back and asked when he could pick up "his" Jeep. He started swearing at me when I told him it was sold.
And what makes you sure some police station midway between you and the seller will run check the gun for you? And if there's something funky about the serial numbers on that model, how will another cop know whether or not the gun is stolen if the cop you talked to can't tell from the serial number? What's a "partial match" on a serial number? Serial numbers are unique ... either it matches, or it doesn't.
Your expectations are unrealistic. If I were the seller I'd already have told you to take a long walk off a short pier.