I think you can do well in the current market, because ARs and parts are superfluous. The beauty of the AR is that anyone can put one together. The biggest problem with buying a used AR is that anyone can put one together.
When buying used, try to get as much information as possible. Your first question should be whether or not the complete rifle is a factory build, or if it is a complete home-build, or if the upper was bought as a complete, with the lower built at home, etc. If you are looking at a home-build, you should ask what parts were used to build the rifle.
Me, personally, if someone tells me they used a no-name parts kit, or a DPMS parts kit, then I automatically fast-forward to me replacing those parts with better components out of my own pocket. If they tell me that they themselves built the upper then I will poke and prod a little more to make sure they know what they are doing.
I'm just tossing out that warning because I feel it it something worthy of consideration when looking for a good rifle at a good price. I have purchased a complete upper from Palmetto State Armory, who may revere, only to receive something that wasn't assembled with a a suitable level of knowledge and/or care. When I shared this information, members here chastised me for it, even demanding that I list part numbers, but the proof is in the pudding.
For these reasons, my usual answer to someone looking for an AR carbine in the current market is to go cough up $1k and buy a Colt 6920 at Wal-Mart or a local (& competitive) store. You think you want a cheap, entry-level rifle, but once you're more familiar with it, you'll realize that this format is one heckuva emergency rifle. Once you've acknowledged that, you might regret buying a lesser, questionable rifle just to save $200. On the other hand, you might be one of those guys with a crystal ball that says "You'll only need it to work for 3 rounds according to FBI statistics..."
