I re-use bar tape all the time. I operate six or seven old road bikes. If/when I change them up I will unravel the tape and, if it's intact, will roll it up to use another day on another bike.
And...truth be told...bar tape doesn't need to be "bar tape" at all. I sometimes make my own out of materials/colors that I prefer.
But...reselling the tape you used as "new" doesn't seem fair since it isn't in "new" condition...unless you priced it as such.
Dan