Collecting photograph slides has been very interesting. You may have a stack of family slides in trays or carousels. Oddly enough these are frequently on EBay in huge lots of 100-1000 for under 20 bucks. Single slides are $1-5. You may have to pay more with buy-it-now. Some are boring travel slides, but on closer inspection some gems can be found. For example pics of Bob Hope at a USO show, rare airplanes, early photos of the World Trade Center, theme parks, parades, football games. Everything! Slides were and still are used of promotional purposes too. TV shows, movies etc, premieres. Some of these still bear studio or photographer copyrights so it may not be wise to make copies for reasale unless you have express written consent to do so. This may annoy a number of people, but a lot of studio promo slides and transparencies are purchased in which mass quantities of  b&w and color 8x10s are sold for 5-20 bucks. Another article can be written about this later.  Interesting slides I have collected:  Actor Wilmer Valderama at a premiere, US soldiers offloading a truck in Vietnam, 60's Dinosaur theme park with huge T-Rex statue, 50's autos, TWA airlines, and a few interesting events ie Rose Bowl Parade and game (60's).  I collect slides which I believe to be the only copy around. Key phrase "the only copy around".  This is still an untapped area with a weatlh of interesting images. It only takes a cheap garage sale projector & screen and a little patience to filter out the slides of peoples deceased aunts and the old family pet.  Hope everyone finds some great images!