I would say that one key symbol within the story is the Headless Horseman himself. He's going to exist as a character and a symbol at the same time. I think he is symbolic of Ichabod Crane's greed. Crane is a greedy character. He wants to be rich and powerful, so that he can be secure in life, and so he can stick it to the people that have been mean to him. Irving wrote that the Headless Horseman was a former Hessian mercenary. Mercenaries usually are symbolic of greed and obtaining wealth at any cost. The Horseman symbolizes that Crane's greed is what is going to kill him in the end.
Crane's greed is symbolized in another item from the story. Food. Crane is always eating. He's always gorging himself.
" . . .for he was a huge feeder, and, though lank, had the dilating powers of an anaconda;"
He eats as much as he can, but he's never satisfied. That's essentially what greed is. Wanting and wanting more and more, but never being satisfied with what you have.