I used to work for a company that paid me to do 'Retail Arbitrage'. I'd drive a Mercedes Sprinter around and spend (On average) between 4-6 hours at a time in a store and, assuming it was fruitful, anywhere between $2,500 - $5,000.
My personal record was at a Toys'R'Us; 9.5 hours (I was there from open to close basically) and spent just short of $22k.
We cleared 1.5m of profit in Q4 of 2012.
I had to quit though as my parents got ill and I had to take care of my younger siblings.
EDIT: Amazon FBA, btw.