If it is a Mk I, I would think a price in the vicinity of $2500 to actually $2700 is fair considering the condition. You might find some cheaper but I would make an educated guess that they've been used rather extensively. It all basically comes down to how much the lens has been used.
If you have the option to shoot with it before you purchase it, of course you will want to listen carefully to the motor for any squeals or noises. It may not be as quiet as the Mk II but it should shift back and forth fairly smooth without out much noise at all. At least that's how mine functions.
Visit some of the reptuable sites and compare their prices. You can never go wrong there.