I buy cars w/about 30,000 miles and keep them forever. Every time you buy, you incur sales tax.
Example, current vehicle purchased for 11,000 w/30,000 miles. We are now at 185k, 14 years later. It's still worth 4k. If we estimate 4k for repairs over the life of the vehicle (new engine at one point), depreciation/repairs is 65 per month. If we look at pure depreciation, $42 per month at this point.
With your six month flip model, you would get eaten alive by sales tax in my state.