I've seen people around here at BF pooh pooh Bicycle Blue Book as complete nonsense, but I've used it before to help negotiations with a CL seller. If your seller is informed and has reasons to disregard BBB, it won't help, but most CL sellers are not that informed, so I'd give it a try, send him the link for the year you're looking at as an argument that your offer of $450 is generous, or at least fair.

You can also try to call his bluff; if you believe $525 is really overpriced then you can say "no thanks, but if you change your mind let me know, I might not have bought a different bike yet". If it sits on CL with no offers for long enough, he might come back.

For me $450 is a fair offer. My first road bike buy was I think $425 or $450, it was a similar year/spec Trek. It was only 8-speed, but it also came with a spare wheelset.

For $525, that bike better be in tip-top condition: brand new tires, tape, cables, housing, cassette and chain less than 1000mi use, BB smooth, shifters dialed in, wheels very true, and most importantly, a perfect fit for your body. Maybe some nice extras like Lights? Computer? Pump?