Vintage photographs are a hugely collected category. The sell through rate is very high. In some categories it is as much as 80%, and many vintage photos sell for over $100 each on eBay.
As with any product the trick is finding out what sells and then finding the product to meet the demand. Neither of these are very hard but they do take a little bit of effort.
As for what sells. Celebrities and Hollywood stars are probably the best, followed by Transportation and the Military. This would include photos of pre-1980s movie stars, famous politicians, early cars, trucks, trains, gas stations, planes and any World War II, Korean War or even Vietnam war photos.
Old photos of towns and cities sell very well as well as anything related to cowboys and indians. Some other categories that do well are photos of large construction projects such as the Golden Gate Bridge or large dams (Hoover, Grand Coulee, etc.), photos of early blues, folk and rock stars, early (pre 1980) shots from the US Space program, any major historical event, and original prints by any famous photographer.
So where do you find these vintage photos? They are literally hiding in the garages and attics of America.
One fellow I know just goes to garage sales. He rarely sees any photos for sale, but he simply asks the person having the sale: "Sir - do have any old photographs for sale?" He may hit five or six garage sales before someone says yes, but he has made some tremendous finds including a box of Hollywood publicity stills of various movie stars from the 1950s and 60s. He paid $100 for the box and sold them on eBay for over $12,000 in a period of four weeks. He also scanned some of the better ones and resells the digital images for $4.99 each.
Photo by ClassicPhotos.