Until the 1980s, the Leopard was considered one of the best tanks available. First built in the late 1960s, it was the first post-World War II German tank design. Although a contemporary of the American M-60A3, the German tank was considered superior. For this reason, Germany was able to export Leopards to many nations, including Australia. Most of the 4,744 produced (plus 1,741 Leopard chassis adapted to other uses, like recovery and anti-aircraft) have been retired (in storage) or scrapped. Australia may have to melt it's Leopard Is down, for there's not much of a market left for 44 ton tanks, even those equipped with a lot of nifty upgrades. The other original buyers of Leopard I have already flooded the market.
The export market is further clouded by legal restrictions, as Germany still retains the right to reject any buyer for the tanks. Thus prospective buyers must pass muster with German public opinion. No nations suffering from bad PR need apply.
The second hand Leopard I market is made worse by the availability of the Leopard II. This is basically a contemporary of the U.S. M-1, and often considered superior to the M-1. But the M-1 has an impressive combat record, and no Leopard II has ever seen action. Still, on paper, and in training exercises, the Leopard II has been impressive. Some 3,000 Leopard IIs are out there, and Germany is still marketing them. Many surplus M-1s are available, and a few Leopard IIs. Add to this the thousands of late model Russian tanks available, and it's no wonder the Australians are having a hard time finding a buyer for its well-cared for and up-to-date Leopard Is.
Australia recently discovered that the market for surplus Cold War tanks has pretty much disappeared. At least for the older tanks, like the 103 Leopard I tanks it is replacing with rebuilt American M-1s.