Why are you upset? Your opinion was that it needed a tune up and would be a good entry level bicycle. You say it sat on CL for weeks. Why didn't you buy it to flip? The buyer that is flipping it, may not even be a bike guy.
There are items (including bikes) that I'll buy on Saturday and sell on Sunday. Doubling my money is a low to poor return on my investment.
I am amused by the CL police (not necessarily you) who think they need to complain about pricing and immediate flipping. The value of any item is what someone will pay for it, so the general opinions of experts doesn't really matter. A young student on a budget is focused on price first. A hipster geek with a wad of cash cares about looks and what the other hipsters think. I can sell to both and make good money. Neither are concerned about where or when the bike was built.
Many CL sellers do NO research on the value of their items. They want that crap out of their garage. Not my job to educate them. I buy cheap, sell high and turn quickly.