You could just leave them the way they are if you want to do an accent wall. If you want to use a sealer for protection I would use the Rustoleum Water based Matte Polyurethane.

Another option is to paint over them.

I would think that refinishing would be pretty straightforward--remove nails, sand, fill voids if you like (but remember that wood filler won't stain the same), stain, coat w/ poly/varnish/tung oil.

I agree w/ sloyd, you might consider painting. The lines of the boards, and even the knots, would show through, and it could actually look great--sort of "refined rustic." Like those painted floors, only on the walls. That would probably be the least expensive and the least work.