Is using WIC to obtain free formula, and then selling it on craigslist illegal?

I thought it would be, but I was just on craigslist and see a TON of ads for good start gentle formula (which is the brand they give her in m) and people are selling 20 cans for $200 or 10 cans for $100 etc.. i am not sure if its from wic but why would someone buy 20 cans and then turn around and sell it for less then they paid? i am not going to turn people in because i am not wasting my time with it, but this just bothers me! its so wrong.