When I had my 300 f2.8 IS for sale, I contacted Adorama. I emailed them a link to detailed photos of the lens and a description about its condition. The lens was 2 yrs old and in mint condition. I didn't want to send them the lens until I got a ball park idea of what they would pay. The lens included the case and all accessories that came with it, all in perfect condition. Adorama told me for a lens in this condition $2500 is the max they would offer.
I have no idea how other resellers calculate the amount they can offer, but at Adorama's used department get upwards of 1,800 pieces sent in for evaluation every week........that's a LOT of used gear (there are around 26-27,000 used pieces in our warehouse right now!) The way the value is established is by using a calculation which takes into account the number of exact same units (of the same grade) in the warehouse at the time of your trade; the average number of that exact same unit in stock over the past 6 months; the average selling price of the unit over the past 6 months and projected decreases in the pre-owned market in the coming months due to new models becoming available. That gives them the ‘value’.
Then they offer up to 70% of that figure, (to cover shipping, insurance, staffing, advertising, the warranty, warehousing costs AND the boss expects them to turn a profit), but of course, a final figure can only be given once they have had the opportunity to physically appraise gear. They send out a pre-paid shipping label so units can be sent in for a thorough evaluation – fully insured – at no cost to the seller! And as you know, if no price can be agreed, there is the cost of returning the gear to the seller.