I wouldn't put too much faith into what the dealer tells you since they often aren't really savvy about EV's and their main motivation is to sell cars.
Do you know where the car is from? The worst thing to do to a Leaf (or any EV really) is to store it at high temperatures and very high SOC. Very low SOC (< 10%) is also hard on the battery but most people will plug in at that point rather than let the car just sit.
If the car is from a hot area (AZ, FL, TX, etc) it is possible that those readings are accurate. If so, you can gamble that the battery will hit its warranty replacement level before the warranty expires but that is a risk. In that case, you can make a strong argument for a lower price.
At 83% SOH it should be dropping to 11 bars very soon. It is also possible to 'reset' the BMS which will temporarily restore the car to 12 bars until the BMS has time to recalibrate itself to the existing battery conditon. That is a pretty shady practice and I would be very leery of buying a car in that condition.