Please tell us what the model of gun is
With all original parts and 95% original finish, plain barrel = $625; matte rib = $825; vent rib = $950.
That would depend on when it was made and what grade it is. Price range could be from a few hundred to a few thousand or more.
Value depends on condtion, how bad you want to sell it, how bad someone wants to buy it, what grade it is, any accessories, the box, original papers, etc.. You could be in the 200-1000 USD range.
No. I visited the Browning Web site and found that the only caliber they had close to the 280 was the 270. Of the moderate priced rifles a 280 can be obtained from Remington, Thompson Center and you might be able to find one in a Winchester Mod 70 Lightweight.