3) When you find out what your buyer wants, put up a trade offering a straw hat, requesting the item you are supposed to heirloom along with however much gold you're getting payed (This is done in the marketplace, located in the equipment menu. Bottom right button in the square box on the left).
4) They trade it to you, you heirloom the item on your retirement screen.
5) They put up a trade offering a straw hat, requesting the heirloomed item. You accept the trade, and you're done.
3) When you have a bid that satisfies you notify your high bidder, then put up a trade offering your item for whatever items/gold you and your buyer agreed to (This is done in the marketplace, located in the equipment menu. Bottom right button in the square box on the left).
4) Message your high bidder stating that the trade is up.
5) Wait for the bidder to accept the trade.