The 22124 was a really nice hybrid with large cabinet mounted trunnions, solid cast 12" wings, and a great commercial Biese fence....the one you showed has an aftermarket Sharkguard splitter on it too (~ $200 new). It looks to be in excellent condition. These saws had a good track record, and enjoyed a large happy following from ~ 2004 til around 2009ish when they replaced it with the granite top 22116 that had a riving knife. $600 isn't a steal, but I think it's fair for what he's offering. You could offer $550 and leave your contact info.
The R4512 has a more checkered history, a lesser fence, steel wings, a lot less mass, and a smaller motor. On the plus side, it has a true riving knife. $350 isnt' a great deal on a saw that sells new for ~ $550, but $275-$300 is fair if you think that saw will suit your needs. The first thing I'd check is the blade alignment, and make sure it doesn't shift when you change the blade height.
Here's an older Powermatic contractor saw in nice shape....I think it's a late PM63 or maybe an early PM64 with a Vega fence, grated cast wings, old school outboard motor, and a link belt for $400 bucks....again, not a steal IMO, but a fair price. I doubt the Marathon motor is original, but it's a good motor.
Worth a look and maybe an offer if it checks out, if you don't want the 22124. https://stockton.craigslist.org/tls/6104416335.html
Once you push the price envelope far enough, I'd be considering something like the Grizzly G0771Z.
For more money, there's also a PM66 3hp for $850 that looks like a really nice deal if you have 220v https://modesto.craigslist.org/tls/6062422170.html
Another nice offering - an older Jet CTAS with a Biese type fence for $500 https://sacramento.craigslist.org/tls/6097022294.html
An older Delta contractor saw for $250 - I don't love the fence on this one, but its worth the asking price IMO. https://sfbay.craigslist.org/eby/tls/6103587561.html
Geez, your area is loaded with 220v saws. 3hp Unisaw with Unifence for $600 - https://sfbay.craigslist.org/sfc/tls/6101822719.html
And the list of nice deals on 220v saws grows - 3hp Grizzly G1023SL for $600. https://sacramento.craigslist.org/tls/6106942969.html
3hp Grizzly G1023ZX for $500 - https://sacramento.craigslist.org/tls/6068453356.html
Her'e a Delta contractor saw with a Unifence - In nice shape, but $550 is more than I'd pay. $400 would be a good buy on it IMO. https://sacramento.craigslist.org/tls/6097040195.html
Here's an older Ridgid contractor saw - $375 is a little steep, but $250-$275 would be a nice buy IMO - https://sfbay.craigslist.org/sby/app/6089565631.html
If you buy a saw that needs a fence upgrade, this Shop Fox Classic is a solid deal at $175. https://sfbay.craigslist.org/eby/tls/6106117596.html