The Poe Salesman is a spirit (or spirit man type thing) that lives in the building right before the Castle Town Market. This is the place where all the pots used to be when you were a kid. He buys poes from you, and when you sell him all of the Big Poes, he will give you a bottle.
You can find Big Poes scattered around Hyrule Field. Chase them on Epona and shoot them, then catch their spirit in a bottle.
nobody knows what or who he is, not even the Sages, Link, or Nintendo. He could possibly be a demon, using proof from his quote in OoT: "Because of the great Ganondorf, it is a fine time for a business like this... heh heh heh... Oh...I can only hope the world gets even worse!"
He's a weird salesman that sells poes