What does Mr. Wilson want to buy from Tom?

Your question obviously relates to the novel The Great Gatsby and refers to George Wilson speaking to Tom Buchanan on his and Nick Carraway's visit to the Wilson's garage. Tom had told Nick that he wanted him to meet his girl Myrtle Wilson, George's wife.
Mr Wilson becomes quite excited when he sees Tom. Fitzgerald describes his reaction as "a damp gleam of hope" springing into his eyes. The reason for this hope is soon revealed when George asks Tom:

“When are you going to sell me that car?”

Tom informs him that his mechanic is still working on the vehicle and that he will provide the vehicle by the following week. Mr Wilson is clearly annoyed by the delay and remarks that Tom's man is slow. At this, Tom warns George that if he should have an attitude about the matter, he might decide to sell the car to someone else. Wilson is clearly unnerved by Tom's dismissive attitude and tries to explain himself but fails to do so.
It is evident that George Wilson is desperate to get Tom's car. The reason becomes obvious later when we learn that George was planning for him and Myrtle to move away. He needed either the car itself or the profit he would make from its sale to someone else, to execute his plan. He decided to move because he suspected that Myrtle was involved in an affair with a man he didn't know.
After Myrtle Wilson's tragic accident, we discover that the car George was referring to is a blue coupe. Tom mentions the vehicle specifically because a witness had seen a yellow car knock Myrtle down, killing her instantly. Tom clearly wants to protect himself from any suspicion.