Here is a simple guide:
Collector's often ask how dealers decide on prices to buy and sell. Here is a little insight into the process.
A customer enters the dealers store, (storefront, show, online), the dealer will pull out his most current copy of the Coin Dealer Newsletter, commonly referred to as the Greysheet. He will look at the bid/ask price of the coin(s). Now depending on his need (inventory-ability to sell) of the coin a price will be quoted.
As an example if a dealer is offered a strict MS-63 1922 Peace Dollar the bid is currently $21.00 / ask $23.00 Now most dealers have plenty of this issue in stock, so the offer will probably be $17 or $18 with a resale offer of $25.00
Now if the same customer offered a strict MS-63 1921 Peace Dollar (key in the series) the current bid is $267.00 / ask $295.00 An offer of $275 to $285 will be made. A resale price of $320.00
It is always easier for a dealer to sell a key date. Most transactions of a dealer are to other dealers. We use bid/ask prices (greysheet) for this purpose. It is used as a reference for retail prices also. Most dealers try to work on a 20% rule. This will cover our expenses of processing the coins for sale.
Now I suggest that every collector obtain a copy of the greysheet on a semi annual basis. (every 4-6 months). This will allow you to better judge your coins value to a perspective dealer.