A fair price is what the buyer and seller agree upon. I priced the scope I bought (or anything on ebay, for that matter) based on what I can buy it for new, and discount appropriately. I saved about 30% over the best retail price I could find. For checking the quality of any scope you might choose, do a google search for it, and there are usually some ratings available, typically at the birdwatching websites.
HTH