Penny Black

The Penny Black was the world's first adhesive postage stamp. It was issued by the United Kingdom on the 1 May 1840 and used from the 6 May the same year. All London post offices received official issues of the new stamps, but other offices in the country did not. Some post offices like in Bath, offered the stamp unofficially after 2 May. 
Once the stamps were used, they had a red cancellation. However, it was hard to see, and it could easily be taken off. It is not very rare. The total print run was 286,700 sheets with 68,808,000 stamps. A mint condition stamp might fetch £3–4,000.