For what it's worth, I bought 3 of my 4 bikes new and I feel a little silly about it now (although I ride a 48, which IS hard to find used). There are some great deals in the used bike market, especially if you are anywhere near a high COL city. I follow two facebook "used bike marketplace" groups, one for each of the two nearby cities -- check around and see if there are any of those near you. I've found it to be a better marketplace than craigslist.
A 52 may be a little harder to come by, but some should pop up, depending on how you define "high end." I'd say the market in my area is pretty full with midrange bikes as people try to upgrade - true high end bikes are a little more rare, but do pop up as well (although those are never discounted as much). You really, reallly, really do not need a truly high end bike unless you are racing, though.
What bikes are you looking at? If you're not sure, you could test ride at local shops, and then look online once you know exactly what you want.
Also: people are probably overpricing because they anticipate some negotiating. Don't be afraid to lowball.