I think the method has a ceiling because if the arbitrage gets to be a certain % of sales at amazon, Amazon will change its rules to make it impossible to make money at it. Why? Let's say Amazon sells something for $50 and you arbitrage it and sell it at $60. So why would Amazon care since they're making money at it, right? Let's look at the numbers. Amazon takes 20% cut when I sell something there. So for that $50 item, amazon takes $10, out of that $10, they have to pay for 2nd day shipping, which probably is like $8 for them so they make about $2 gross profit. Now, if they cut the arbitrage people out, they sell the item at $60, get a $12 cut, pay $8 for shipping, and now they made $4, which is double the profit for Amazon. I suspect that right now, they tolerate arbitrage because in increases seller count. But at some point, they will say, "hold shit, we could be making $XXX,XXX,XXX dollars a month if we cut out arbitrage".
At that point the bottom 99% of arbitrage people will be put out of business.