Fuel tank warps and leaks at the filler
The plastic tank warps around the filler neck and slops fuel onto the bike and your legs when full — 2016-2017 bikes were recalled (17V388000), and NHTSA complaints show even replacement tanks warping again. Fill it up, ride briefly, and look for wetness around the filler lip. Run the VIN for the recall and inspect the tank flange closely before paying.
Ask for repair records covering this item. If it hasn't been addressed and the car is in or near the mileage window, price the up to $900 repair into your offer — or make it the seller's problem.