I think each gun has to be evaluated on it's own plus' and minus' and pitted bores are always a minus. As far as shootability, to my mind the degree of pitting has to be weighed against the barrel wall thickness as a semi objective measure. The thicker the barrel walls the better. As far as pressure, pressure is highest at the breech and drops rapidly (in a few inches) as the shot charge travels down the bore. Remember Sherman Bell shot overloads near 19000 psi in 15 "wall hangers" some with severely pitted bores and none blew. Of course you don't want to find the first one that does. One of my favorite hunting guns has pitted bores, they take more time to clean but they shoot fine. If you really like a gun otherwise and the barrels are good and stout I would not pass on it if I liked it. I don't know what the discount would be for pitted bores but one idea is to find out what the going rate is to have them cleaned up by someone like Brad Bachelder and use that as a negotiating tool trying to get the seller to discount all or some of the barrel work.
I think Chuck H has a good point on the DoubleGun board, look at the highly engraved Fox in the picture, as he said nobody worries about the deep pitting ;-) on the outside of the breech of that Fox.
http://www.doublegunshop.com/forums/...=268477&page=4