There are people out there that have actually found this sort of treasure and sold it on for a vast amount of money, but these finds are few and far between. As there are thousands of coins issued at any one time then the likely hood of finding one that is worth more than a few pounds is very scarce.
This is why some people have come up with online journals or blogs which spur budding coin enthusiasts on. There are not only journals for old coins which have been found by ordinary man, but for new coins which have been issued too. These give an incite into how many coins have been produced and what peoples thoughts are on a particular coin.
An online journal is a good thing if you have a particular coin which you are wanting to sell. A journal could give an incite into the best places to sell this coin and the shops which offer a good price for coins (Münzen). There are many online shops selling coins (Bewertung Münzen) and most of these shops offer to buy certain coins such as rare euro coins.
There are certain auctions which are offered through the year in which you can go any see rare finds and find out what collectors are bidding for them. A diary of coin auctions can be found listed on the web, along with the cities which play host to these auctions.