If a man purchases a pony for $60, sells it for $70, repurchases it for $80, and sells it again for $90, try to move toward every exchange as discrete:
-60 + 70 = 10
-80 + 90 = 10
The man-made $10 with every exchange, hence his benefit amounts to $20.
Did you move toward it abruptly? Tell us in the remarks, and Offer this issue to test your companions.