Sleeved barrels are the only way to make many guns safe to shoot. You dont sleeve a gun until you have no other choice. As to value going from wall hanger or scrap to a shoot able gun does improve its value. Not to pristine condition but to at least a decent percentage.

If a gun has thin barrels it will be very hard to sell period. Who is going to buy a gun which they can not shoot? One wall hanger per person is enough. Sleeving saves guns from the closet or scrap heap.