Devin is buying 12 for $12 so for each he bought it was $1. If he’s selling them for the same price he bought them for (each) when he sells all 12 for $1, he will be left off with the same price he started off with. Devin is not making any profit. What i would do is sell them for $1.50, and then after each is sold, the total earnings would be $18.