Treasure hunting begins at home. People regularly find famous old books and valuable antiques in their attics, so get up there and look. One couple opened their walls while remodeling, and found them insulated with thousands of old and valuable movie posters. In the thirties a theater owner ahd put them there. The couple sold $200,000 worth of the posters, the last I heard.
As a general rule, online coin dealers are far more likely to accept credit cards than your local “>coin shop Pinnacle in the crawl space under our house. Hotel owners report that visitors tuck money in the bedside bible, and forget to take it with them. Sometimes you just have to look.