+1, looks like a 56cm. Actually, I believe that is the 1983 model, as it only has one set of bottle bosses. The rims are replacements, as are the saddle and rear derailleur. The brakes appear to the OEM Dia-Aero Gran Compe 300 series, as does the crankset. Overall, there's nothing to suggest the cahnges are actual upgrades, so they would actually deteriorate the value IMO.
Still, the frame is a nice one, made of lightweight Tange #1 and they are fairly rare. If you're in a high priced market, $300 may be fair. This is not a good time to be selling bicycles, but it is the most size. You could try offering him less and see what happens. FYI, orignal price was $535 US.