Re: 27" Apple display 2010 for $400
It certainly _could_ be a good deal depending on quality and condition — that display cost $1000 when it was new. $400 is less than I see them on eBay.
If you don't need some of the features of the Cinema Display — the built-in MagSafe power adapter, USB ports or mini DisplayPort — you can get a much less expensive 27-inch LED display new, from reputable online resellers like Newegg.com, for a fraction of the price. It won't have an Apple logo on it and you'll need to use an adapter to connect it, but it'll work perfectly well. (I have a 23-inch ViewSonic I picked up a few years ago that's worked great with every Mac I've connected it to.)
I might consider taking a look to figure out a) what gives you the best bang for the buck and b) whether you're willing to chance a private sale compared to a factory warranty on new product.