I've done very well with 35mm cameras, SX70 Polaroid Camera, ANY Polaroid film ($20 to $35 a box!), Lenses, and so on.
I also do well with Vibrators (COME ON NOW!) Stim u lax, ultrasonic, infrared, and so on. They generally cost $2 to $4 each and sell for $50 to $100.
I have done pretty well with Better quality Olympic Stuff. Here you REALLY have to know what you are doing. A souvenir medal will sell for $1, a volunteer medal will sell for $40 to $100, and an Athlete's participation medal will go for several hundred dollars. I did pick up a Bronze medal once from the 1936 Olympics. I was about 75% sure it was real. It turned out to be real, and I was able to Flip it for $8,000. The guy who bought it was only 3 medals away from a Complete set, 1896 to 2008 gold silver and bronze, summer and winter Olympics, plus every torch from every torch relay. He has a better collection than the IOC or Prince Rainier of Monaco.