Wow. Great advice here. I've been buying and selling gear for 40 years and most of my system I bought used or on closeout. Here's a few points that I didn't see mentioned above.
1. Buying used gear and/or gear that is considerably less than full retail takes a lot of time. If you enjoy doing it it's a fun part of the hobby. If you don't enjoy it then it will be tedious and worrisome.
2. If you are looking for a particular piece of gear you can use HIfi Shark to search for everything used for sale at that time. You can also put an alert on eBay that lets you know when that particular piece of gear gets listed. Also, do a Google search for the item with the words "for sale."
3. eBay will let you search for sold items so you can see the price they actually sold for. There is also a way to do this on Audiogon but I haven't used it.
4. If you are interested in a particular piece of gear keep an eye out for the manufacturer introducing a new model. You can sometimes get new gear at a used price. I bought a PS Audio transport and DAC at a 63% discount ($4k each marked down to $1.5K each) when they introduced their new models. I also just bought a new Marantz KI Ruby SACD player for $3k (had been $4k) on closeout. This strategy goes back to late 90's when I bought my Krell KSA 300S amp for about half price (dealer demo) when Krell introduced its new FPB line (I still have this amp).
5. I estimate that I must have bought over 50 pieces of used or closeout gear over the years. Out of that total I think I've received a total of 4 or 5 pieces that were damaged or not accurately represented in the ad. All of the problems have been on eBay. I've never bought on Agon but I've never had a problem with anything I bought on US AudioMart. If it happens to you stay calm and attempt to work out an acceptable solution with the seller. This forum is a good resource for advice if you are having a problem resolving the issue.
Good luck and happy listening!