It's all going to depend on who in your area has a bike they're selling and what it is, but yeah, there's no reason to think you can't find that.
It's one useless data point from years ago, but as an example 2012 when I decided to get a road bike after not having one for a long time I found one on CL that I bought for $475 from the original owner. He'd ridden it enough to mostly, but not completely wear out the original tires that came with the bike. It was full Ultegra. It wasn't the then-current Ultegra, since the bike was 9 years old, but it was in fantastic shape. That was one of many listings.
Btw, don't tell the sellers that you have $800 to spend on a bike unless you want to get a bunch of offers for $800. Chances are good if you look around and are willing to look at bikes with previous-generation groupsets that you can do much better than that.