Answer:
Joseph must sell for $650
Step-by-step explanation:
A contract is formed when there is an offer and an acceptance.
When Joseph made the offer of $650, Chelsea only made an inquiry on the extra tyre. She was now willing to buy without the extra tyre for $650. So the contract has been formed.
Joseph is now saying Chelsea made a counteroffer (which is not true) and he wants to sell at $750.
A binding contract has already been made for $650 and Joseph must honor it.