Always offer the $300. Maybe he will take it, maybe he will say $350, which you already know. Very few of the Colt DA guns, esp in cals bigger than 38 special can be found for this kind of money. Look around - they are in demand.
I'm not sure why this same question was posted more than once on various threads. These troopers are not often seen in my neck of the woods, and for $350, with original finish and stocks, its a buy. Be sure to check it for lock up with the hammer back on each chamber AND then AGAIN with the trigger held back but the hammer down. If you are able to move the cylinder out of lockup at any point, the gun needs re-timed by Colt. Call them and make sure they will do the work. Many of the old models they don't work on anymore.
Winchester 73, the TFL user that won the west