This spectral being is an interesting one. He fought in a battle in the Howling Abyss and was bound there to warn people of coming attacks. He even tells players "Avarosa herself killed me, it was a great honor. She bound my spirit here, so I could sound the horn when the Watchers returned."
So not only was he involved with the battle as well as respected his opponents, he wants to aid other fighters by selling items. While he limits the ability for players to buy once they leave the area, Greyor provides the service which can help players push forward in their future battles.
It's good to know that even in death, a salesman can still flourish and people will still buy without getting freaked out. Or calling Ghostbusters.
Image source: League of Legends wikia