As far as prices, it all depends on the bike.

Build one with Campagnolo Super Record, 11 speed, and if it doesn't exceed $1000, then it will be in really battered condition.

But, prices are falling on used carbon bikes, in part due to building new ones with lesser components.

Here are a couple of local-ish ones for an example.
https://eugene.craigslist.org/bik/d/...754375782.html
https://eugene.craigslist.org/bik/d/...752486499.html

Craigslist, and other "classified" ads are good, and probably better prices than E-Bay.

I usually think of any used bikes I acquire as needing a complete tune-up/overhaul. However, that may depend a bit on whether you're shopping in true bargain basement territory.