Used postage stamps can be sold for cash if they are sold to collectors. However, the stamps should either be rare or very old or should otherwise be determined to have additional value beyond that of the postage value.
The value of used stamps is affected by the markings from the cancellation process. Typically, as long as the cancellation mark does not interfere with the overall appearance, graphic or design of the stamp, then many stamp collectors will purchase them. If the bulls-eye portion of the cancellation mark, which shows the date and city where the mail was processed, is in the middle of stamp, it may be worthless.