if the tranny does go boom,,the repair cost will be at least 1/2 of what he is selling it for.
If you are really interested in buying it,,take the car to a mechanic of your choice and tell the mechanic you are thinking of buying the car and you would like him to give the car a complete inspection.
A word of caution here,,do not go to a mechanic that the seller recommends,,the bias here should be obvious to you.
If everything looks good with the inspection you may ask the seller to split the cost of the inspection with you.
Now if the car was taken care of it should go another 100k.
Of course upon your delivery you should have the car completely serviced.