A symbol is, simplistically, an object or idea that represents something else. Therefore, anything which alludes to "it" existing as something else is considered a symbol. A symbol acts as a metaphor in most texts given a metaphor is a comparison between two, normally, unlike things.
In regards to Walter De La Mare's poem "The Listeners," symbols are only easy to find if the reader engages in the text. A reader who does not engage in the text does not look deep enough to find hidden symbols. The reader simply reads the text and does not examine or analyze the text.
Based upon this, readers can make symbols out of almost anything. If a reader finds that something in the text forces them to reexamine the words, objects, or images for deeper meaning, then the text almost assuredly contains symbols. Herein though lies a problem--not all symbols can be agreed upon. Some may not see something as a symbol, they may not agree with another reader's interpretation of the symbol, or they simply may disregard the idea as irrelevant.
Therefore, the symbols mentioned here are ones based upon my own response and interpretation of the text.
1. The door: The door symbolizes the boundary between world of the living and the world of the dead.
2. The bird: The bird represents the one being who can cross the boundary between the world of the living and the world of the dead.
3. The traveller's grey eyes: The grey eyes of the traveller represent the unknown and blurred world he is entering.