How can a majority business-use vehicle that I sold to CarMax for $8k and also had to pay an additional $2k cash to satisfy difference in payoff result in a gain?

Put into use in 2018. Drove 500 personal miles that year, which was the only period of personal use for that vehicle. Drove approx 73k miles after that brief personal use. Used standard deduction prior 4 tax years. Did not have a trade in at any point. Used actual deductions this year due to purchasing cargo van for work.