I don't really do much prospecting but I have scored some nice hits on miscategorized items on Ebay:
I got this Benny Hill autographed PC at a postcard show where the seller not only sold it for a pittance but showed me that it was signed when I was checking out. Whatever...
Same seller had no idea what this was and let it go for 10% of value:
Best of all is when I know something and hope like heck that the rest of the world misses it--and they do. When I saw this John C. Heenan CDV I knew it was something special because I'd seen an Abner Doubleday with the same kind of copyright notice on it. I did some research and discovered that the copyright notice is pre-1870 and that the card is in fact the earliest known commercially-issued boxing card, made and sold commercially before 1870:
