What you do is prepare a sales package with a copy of the lease and permit, the address of the sign location, some photos with red arrows drawn on showing where the sign would go, some reasons why it's a good location (traffic count, near some landmark, etc.) and priced as "best offer". Send it to all the billboard companies in that market. See what they offer on it. Try to get two that are interested, and play it off between them trying to get a higher bid (just like eBay). Pick the winning bid (not always the highest, as you still have to get someone to pay you and the bigger company may be a safer bet than the small guy) and try to get it closed and funded.
It's no different than selling a car, except the price is much more negotiable probably.