Same as @
Geekage, I re-wrap with used tape if I have to get at my brake cables or some other emergent circumstance where I need my bar naked but don't have new tape. If I am planning a larger repair where I will need the bars unwrapped, I take the opportunity to buy and install new tape.
I can't imagine anyone buying used handlebar tape - that's kinda gross. Just keep it in a ziplock bag with your other spare parts and you can re-use it if the need arises - ie. tape is torn or damaged in a crash or when trying to unwrap to get at your cables. Or if you are planning on selling the bike, the fresh tape will make it look like new, even if you don't like it.