What can I give the pawn shop owner for a trade?

Hint 1 of 5:

Actually, you really can't trade for anything.

Hint 2 of 5:

But you can buy one of those items.

Hint 3 of 5:

Have you looked on the beach yet?

Hint 4 of 5:

Try moving one of the planks to see what's underneath it.

Hint 5 of 5:

Open the box and look inside.  You'll find a coin.