Gussyboy1,
I just tried this with a 3 cent USA star self adhesive to see if it still worked as I remembered. This is a hazardous solution, so do in ventilated area with some gloves. I used to use Ronson lighter fluid as it is a little less hazardous, but not having any on hand I used acetone ( from paint department) and not the "acetone" fingernail remover

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I go into the ventilated room with thin gloves, pour just enough acetone to cover stamp in a glass saucer. Let it soak about 30 seconds and you should be able to lift off the stamp. I use a wooden toothpick or the pointed end of a disposable floss holder. Throw away the piece of paper the stamp was on and re-immerse the stamp , gum side up, and I use a small piece of paper towel or q-tip soaked in the acetone and wipe off the glue gently. Then carefully put the stamp, still gum side up on another piece of paper towel and when the acetone evaporates, the back should be stick free. If small area still sticky, repeat the last part. The Ronson lighter fluid also does a good job of watermark detection in my experience. Now that I realize I don't have any , it is time to visit the 99 cent store :) WARNING : both fluids are flammable. Don't use near a flame such as gas stoves or hot water heaters. Please be responsible and if not an adult, obtain adult supervision!
Jim