Is stainless steel a problem?
I wouldn't think so. Some grades of stainless are gummy, hard to machine and they gall easily, however the high carbon stainless used in gun barrels should be just about as workable as carbon steel. I'm not a gunsmith or metallurgist, but that is my take from working with lots of different kinds of stainless in food industry maintenance.
As for the fix? I would imagine that drilling and tapping the hole to the next size screw would be the "right" fix but might also require drilling out any mount used with that barrel to accept the bigger screw. There are also alternative methods like heli-coils and such that would keep the same size screw, but might not be reliable enough under recoil. The cost for either of these fixes should be reasonable. As for ugly, you wouldn't be able to tell it was repaired without removing the scope mount.
Curtis
P.S. Forgot to add: If you are considering buying this barrel, be aware that you will need to get a great deal on it. Even then, fixing it probably won't increase the value much if any. Also 10" 30-30 barrels traditionally sell for pretty low prices ($110 to $120 range and sometimes lower) compared to other calibers and even compared to longer 30-30 barrels. Some find the muzzle blast and recoil from the short tube unpleasant in 30-30, and the round is really not efficient in that short a barrel. If it were me I'd give $80 to $90 for it, tops.