I posted here last week about purchasing a bike, and I'm back again. I'm 6' with shoes and pushing 280 lbs. I've ridden the new Diverge, Crossrip Elite and I loved those bikes. I just today ran across a 2010 Marin Cortina CX 56 for $400 (asking price). This is from the ad "Sram Rival shifters, Sram rival front derailleur, Sram Red rear derailleur,12- 28 cassette, FSA SLK Light cranks, 2013 DT Swiss 3.0 Etrto wheels, Specialized Tracer Pro 33mm tires, Avid Ultimate shorty cantilever brakes, FSA 3D forged stem 100mm +/- 6 degrees, FSA Wing Pro bars, specialized carbon fact 2 bolt post, Selia Italia XR seat".
I'll be using the bike for paved as well as gravel roads.
Is this a good bike for a guy my size, and is that a good price?

As with all things used, the value depends on the condition. Those wheels are pretty lightweight and low spoke count for a person your size. I'm not sure I'd trust them on paved let alone gravel roads.