Because dealers expect to make a profit, you should expect to get less than catalogue. If you want a valuation, make it clear that you expect to pay for the service. Most dealers specialise, and may not be interested in your coin. Coins are produced in vast quantities. For most modern coins, mintage figures are frequently hundreds of millions each year.
Nobody ever throws coins away.
So millions of old coins are lurking in old drawers, biscuit tins, under floorboards, mattresses, buildings. This is good news for coin collectors, because they can buy even ancient coins for a few pounds, whereas a contemporary artefact which the coin would have bought may be worth 100 or 1000 times the value of the coin. There are many more coins in existence than there are coin collectors. Most coin dealers started out as collectors, and have a genuine interest in coins. They will often help those with a genuine interest in coins.