Shel Silverstein wrote, "If you are a dreamer, come in. If you are a dreamer, a wisher, a liar, A hope-er, a pray-er, a magic bean buyer, If you're a pretender, come, sit by my fire. For we have many a flaxgolden tale to spin." What is the meaning of “For we have some flax-golden tales to spin,” and why did he use this specific wording?

Flax is a plant that has flowers, but it's even more well known for its seeds. They come in brown or golden varieties. Golden flax has bright yellow flowers, which is a color that symbolizes the sun, happiness, and hope.
The plant is used in making textiles, as well, such as linen. Flax fiber is soft and flexible, as well as shiny. It is said to look like blonde hair when arranged in bundles. This is where the term "flaxen-haired" comes from.
Shel Silverstein was a poet and a wordsmith. Many of the words in the passage from Where the Sidewalk Ends are invented by Silverstein in some way. Instead of saying optimist, or one filled with hope, he invents the word hope-er. He does the same with the word pray-er. His entire message here is to people who value imagination and storytelling. His words are an invitation to join him in creating worlds on the canvas of imagination.
He uses the words "flax-golden tales to spin" as a type of play on words, and also a phrase filled with imagery. It can be understood as a metaphor, as well. Stories are also known as "yarns." Storytellers were often said to "spin" these "yarns," which is a metaphorical way of saying they were telling a story. They weaved the characters, plot and setting together much like a weaver creates a tapestry on a loom.
It isn't accidental that Silverstein chose golden flax, which he then creatively inverts to flax-golden. This is the fiber that resembles blonde hair, as noted earlier. Blonde hair also resembles threads of gold. This shows the value he places on imagination and also the beauty he sees in creating tales. It is reminiscent of tales like Rapunzel, the beloved story of a woman who was locked in a tower and had flowing golden hair.