Like Don says, it was the Royal Ordnance L7 105mm cannon. For awhile it was the main gun of quite a few tanks, including the first gen M1's, Leopard1's, South Korean K1, M48 Patton, Stryker, US MGS, Argentinian TAM, a Chinese tank and the list goes one for another 10 or so vehicles. Probably one of the most successful and widely used tank cannons.
As for a market for them, people will buy them but they are very easy to find. Most are US ones either in steel or brass. Can easily get one for less than 70$.