Kbb tends to be slightly high.
If your Miata is in such good shape and your listing is excellently explained, the first thought I would have is that it's priced too high, regardless if it's $500 below kbb private party. Market speaks for itself.
I went through this when I sold my 2001 E46 325Ci; I was so attached to my baby that I was unrealistic about her true market value. Once I got with that, she was sold in a week.
I recommend look at other similar cars in your area and if their list price is much lower than yours. No one cares about your new tires. They would only care if it had old/bald tires which they'd use as a bargaining chip.
Also take it to Carmax. You don't have to sell it to them, but Carmax is generally reasonably fair. Compare local used Miata listing prices to yours, and to Carmax, and you'll have an idea of the market price for your car.