Sounds like a wheel vibration. Take it someplace that uses a “road force balancer”. This type of balancer will look at wheel roundness, as well as balance. If the wheel is bent or the tire has a high spot the road force balancer will tell the technician. Rental cars take a beating and people don’t always tell the car rental company the truth. Hitting a curb can damage a tire and also a wheel, causing vibrations. Renters rarely disclose such incidents.
I don’t trust those “100 point inspections” done by the seller of the car since they are the ones trying to sell it. If you want an honest assessment of a used car, pay a mechanic $100 or $200 to check out any used car you want to buy. If the seller won’t allow it (or the inspection reveals more problems than the seller disclosed), walk away. There are plenty of used cars out there.