People often ask way too much for used ones as they're Apple, and you can no longer buy an Apple display in that size. It's important to know that they can break. I have personally seen failing power inverters and other issues with them. They're not perfectly uniform, but if it's a good unit without too many hours on it, they can be very nice. There isn't any way to tell how many hours are on them, but you can somewhat gauge the life left on the display via a colorimeter. I don't recall the half life, but if you profiled it at maximum brightness and compared that to the factory specs from a site like everymac or Apple if they have tech specs in the archives, you could get some idea of its condition assuming no parts fail early/prior to backlight death. If it's less than half the factory spec brightness (note the term half life) it's an aging display. You'd want to see a picture of a grey or black screen too to see how bad any potential uniformity issues or backlight bleed may affect things. Some of the 30" displays are way more than 2.5 years old, unless you're talking about the 27".
The Dell U2711 is popular. If you're into photography or graphic design, the NECs are nice as they have more calibration features. You might read about antiglare coatings, but they're the same types of coatings as you have on that 23" ACD. These are however updated panels with better contrast. The 23" ACD wasn't just flat in terms of specs. It had a somewhat weird response, and highlight detail wasn't that great.
