Fungible means the asset is transferable across markets. For example, gold is fungible across multiple markets, you can buy it in London, sell it in N.Y. . Fungibility is a neccessity for a true market arbitrage trade.
Intermarket arbitrage is difficult. You have several variables to contend with, currency rates, liquidity, clearing, etc. You need to have the proper clearing arrangement and and scale to make it viable. The simple answer, if it was simple , and viable on multiple scales the opportunity would not exist because the opportunity for low risk out-sized gains immediately attracts players who arb it out.
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