Wilson has discovered that his wife, Myrtle, is having an affair. He does not know who it is, but it really does not matter. His wife is being unfaithful, and Wilson thinks that moving out of the Valley of Ashes will fix the problem. In all likelihood, moving Myrtle is a temporary solution. She will no longer be having the affair with Tom, but she will likely still be unhappy with her station in life and marriage. Moving Myrtle out west is not likely to fix the marriage. Wilson may or may not know this, but he does know that moving away from the Valley of Ashes will stop Mytle's current affair. It is a brave move on Wilson's part as well, because in some ways he is giving up on his belief that he can strike it rich in the East. However, the move does show that saving his marriage is more important to him than making money.
Wilson has discovered that Myrtle is having an affair with someone, though he doesn't know who. Moving Myrtle out West is his way of dealing with the situation. Wilson actually loves his wife and wants to take care of her. In his mind, taking her away from her lover and secluding themselves from the big city environment is the way to work on their relationship. This reinforces Fitzgerald's disillusionment with the American Dream, represented by the East. Wilson is giving up on the idea of making something big of himself, and moving to a simpler, calmer state of life.
Wilson knowing that Myrtle, his wife is having an affair he decides to move his wife from the Valley of Ashes to the prominent West Egg. Where he may get over her affair and he thinks it may be the person trying to sell him the yellow car.