There are persistent differences in price because it is hard to move the government currency in and out of an exchange. Also, maybe some people don't want to deal with certain exchanges because the risk of the exchange running off with their money is too great.

I had a complex statistical arbitrage strategy (versus pure arbitrage) once that worked for small amounts. Maybe I averaged making $25 a day with a paper trail a mile long. It wasn't scalable, unfortunately. I doubt my accountant would really understand it, didn't want my bank to drop me due to weird transactions and didn't want to potentially deal with the government. It was complex. It involved BTC, buying stuff on Amazon and Nordstrom gift cards.

Small pure arbs could certainly still be done by individuals. I just pulled up coinmarketcap.com and randomly scrolled to Lisk. LSKUSD is currently $2.52 at Livecoin and $2.67 at Bitbay. You have equal amounts of bitcoin and lisk at each broker. Let's say $50 bitcoin/lisk at each. Buy $50 of lisk at Livecoin and simultaneously sell $50 at Bitbay. Then move the coins around so you have equal amounts lisk bitcoin again, and do it again. The difference in price minus commissions and fees to move around lisk bitcoin is the arbitrage profit.

A few points:

1. Bitcoin fees are high. Maybe you can find a different pair with lower transaction fees. Dash might be cheap.
2. The biggest price differences will be between exchanges with negligible volumes. In our example, we used Bitbay which only has transacted $1,557 in lisk in the last 24 hours.
3. You are at risk of bitcoin dropping in value. Maybe you make money on the arb but lose money on bitcoin.
4. What if you get filled on one exchange but can't get filled on another before the price moves against you? You lose.

Scenario 2:

Bank accounts linked to Bitstamp and Gemini. Again, I go to coinmarket and see the bitcoin price at Bitstamp is $2,591.50 and $2586.94 at Gemini. You have 1 btc at each exchange and $2600 cash at each. Sell 1 btc at Bitstamp and buy 1 btc at Gemini. Move 1 btc from Gemini to Bitstamp. Withdraw funds from Bitstamp to bank and then deposit at Bitstamp. It will take days and you will lose money on fees and transaction costs. If you are US, you will get reported for suspicious activity by your bank if you do this a lot or the amount is over $10,000.