If I want to do service business to buy cars for my customers in over seas ,and they wire the money to my bank account to buy the cars for them ( since they pay me my fees $150 for the service ) is the cars money taxable or just my fees ?

Just your fees are taxable for income taxes, but for sales or ad valorum tax, the car money is taxable

This is a business, so you report all income, ( prices of cars) and then show what you pay for the cars as an expense.

Fr Chuck's way is actually the correct way to account for the cash flow.