It's the same with most s/h or collectors items, whether cameras, stamps, coins, vintage cars, or whatever. Selling to a reputable dealer means that they have to price to cover all their overheads and risks while the item is in their stock awaiting another customer, pay their sales or profit taxes ,but you get the cash with no worries or hassle. Selling privately, via Ebay or whatever, means that you spend your time, take any risks of auction bids, and pay any costs and auction commission......in the hope of getting a better return.
There's a big firm who advertise in the UK to buy any used car......their adverts say that they will buy any used car for immediate cash, but admit, quite openly and fairly, that you "may" get more in a private sale. Guess it all depends on what you think your time and effort is worth.