You own shares of that corporation, which gives you a say in how the building is run, and you get a lease that allows you to live in a specific apartment. But when co-op shares are sold, it's on record that they've been sold, but not for what price.
If you want to know more, check out attorney Sylvia Shapiro's book . It's kinda wonky but full of good stuff.
So how do you get comps? You can go to a friendly brokerage agency, because we maintain historical databases. (This is one of the arguments our opponents make when they say we're pricing off of information control.)
You can also go to the co-op's board, or managing agent, and tell them you're selling, and it would help you to have recent transaction info. They may or may not help you. If you've helped your co-op community by serving on the board, you have a better shot.
Or you can collect the information yourself. One of the first things I tell people who are thinking of selling in six months to do is to start collecting newspaper/web classifieds of similar apartments. Both the Times and the Post run not only sales listings, but "just sold" prices. (For the Post site, hit the "real estate" tab on the left to get to back issues.)
These comps are provided by the real estate agents, so they're not always 100% accurate, but that's the case with the info in the brokerage firm's databases too.
Still, it won't hurt to compile it. Start making a folder now.