I would recommend checking out Zybez as it is linked on there. It is pretty much the GE of Old School but is just outside of the game. You can pretty much always find people buying on there.
An alternative is just going to world 1 or even world 2 and advertising Nats for sale. I'd say sell around 240-270 ea. Just depends on the demand at the time!
Good luck, I'm also willing to buy from you. Pm me or add my skype!