Sometimes we treat prayer like a business transaction, as if God needs to be talked into giving us what we want. But the real purpose of prayer is never to change the mind of God. He does not need our better ideas. He does whatever He pleases, and His wisdom is far beyond ours. To think I could present some new information or persuade Him to see things my way would be both foolish and proud. That is not what prayer is for.
When we plead with God over and over for a material concern, the deeper issue is often which mind is ruling us at that moment. The mind of the flesh is fixed on body needs and circumstances, and that kind of mind brings anxiety and death. The mind of the Spirit is life and peace. If you are living after the flesh right now, your thoughts will be completely consumed with selling that sewing machine. If you are living after the Spirit, you can bring the need to God and still rest in His care, whatever the outcome.
God knows you need resources. He accepts a willing mind. But you do not have to strain to change the mind of a potential buyer, and you certainly do not need to change the mind of God. Bring your request honestly, then let it rest with Him. Ask instead that your own mind would be ruled by the Spirit. The peace you are looking for is not waiting on the other side of a sale. It is found right now in turning your mind away from the desperate preoccupation with the thing and toward the Lord who already knows what you need.