Were it me, I would take a range rod & some white patches & some Ballistol & swab the bore real good with it, then run a couple of dry patches down it & check it out good. Oil in a bore or grease will appear to show shiney & etc. and it still could be pitted. I know you say it is good, but appearances can be deceiving with a light. Reason I say that is if the outside is rusted all over lightly, chances are the bore has had the same care. I have look at ALLOT of them & thought the bore looked decent until I swabed it good & found out otherwise.
I found 2 of them up in Ohio a couple of years ago & man they looked Great on the outside & upon bore inspection, found them full of cruds, shot & never cleaned & a total wreck inside. However, the shop owner assured me that ALL ML's were that way & it was "normal" as ML's just get that way because of shooting BP in them..........
As for the price, if the bore is good Buy It and quit haggling with the guy as the first other guy that sees it for that price will take it home.
If ya get it, take some Ballistol & some 600 or 1000 wet/dry paper & wet the paper with the Ballistol & carefully rub the flats & that surface rust will come right off. The reason I was use the paper is because you can cut it into 1" squares & fold them in half & carefully rub it & not rub the brown off the edges of the flats. If ya use steel wool you will rub the browning off the edges.
But that is a real good price if it is in good condition.