Stator and regulator charging failure
The Tuono shares the RSV4's V4 and its charging weakness: fried stators and regulators are the platform's best-documented flaw on owner forums. Test charging voltage at the battery at revs and look for receipts for updated charging parts. Budget for a stator and MOSFET regulator on any bike with no records.
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 $550–$1,100 repair into your offer — or make it the seller's problem.