Not a bad deal. $650 could get you a S&W Sport that I am sure someone will say but then again you are getting some extras too.
$40 optic I think?
Say $20 for each mag assuming they are decent, although you can get them cheaper, so that is $100
130 rounds of .223 is about $45 nowadays just make sure it isn't steel case stuff
Sling, I wouldn't value since you may want to swap it out for a different one immediately anyway.
Hard case, also tough to value but even the cheapo ones go for $30 nowadays.
So you are getting like $200 worth of goodies, if you want to look at it that way, I think $450 for a used DPMS is not bad if it is in good shape, but you could maybe talk him down a bit.
As far as what to look for, aside from the usual wear and tear, I don't know. Never bought one used. See if it has m4 feed ramps, check the fit of upper/lower, break apart and look down bore, take out the BCG and see how it looks, pay extra close attention to the bolt and lugs of the bolt, cycle the bolt, it should return to full battery when released from half-pulled, see how the trigger feels, put in an empty mag and pull the bolt back to verify the bolt catch works.