Old Picture (photos) Postcards are the ones that are worth the big buck$ because of their historical value but they are what I will call “location sensitive”. Meaning they command the highest prices by collectors in the locale that is the subject of said picture.
And postcards being postcards, one might find a old picture postcard from Savannah, Ga, at a flea market in Seattle, Wa, and that is why one often sees someone thumbing through a big box of postcards looking for a “sleeper” that they know where to sell it at or to.
For instance, a PP of a Coal & Coke RR engine and crew might be worth 50 cents in Austin, Tx, but might sell for $100 in Buckhannon, WVa.