Like you said the breech and nipple look nice and so does what tiny bit of the bore that is shown. If its never been shot the bore may be coated. As far as the exterior, who knows where it was stored and how often it was wiped down.
I swapped for a 28ga over under a while ago, the outside of the barrel was pretty rusty, inside was great, i sent the barrel to precision bluing in upstate ny. he did a matt finish on it for 100 bucks. it came out nice, the matt finish hides most of the pitting, looks good and shoots great. like Michigan said,buy it you don’t know if you will see one again
Not my cup of tea but don't believe what I see would scare me any. Not many guns made in that era in "perfect" condition. One thing I've learned especially when dealing with a hard to find object that you need to be willing to gamble a little bit , if it turns out that you can't live with your decision you will most likely be able to recoup most if not all of your investment.
It's costing someone $500 by the time its delivered. Figure another $100 minimum tp have it re-blued or coated. Seems like i could get a rifle sleeved or rebarrelled for less.
Hope it works out well for the new owner.