They certainly can purchase a business vehicle, and bonus depreciation would deduct the full cost of the business percentage of the vehicle (assuming the vehicle is over 6000 pounds).  It is just like any other asset.

But no, selling the vehicle after that WILL result in taxable income, regardless if they purchase another vehicle or not.  That second vehicle may qualify again for Bonus or 179 (but as of now, Bonus is reduced to 80% in 2023), but that does NOT affect the gain of the first vehicle.