Just a few months ago, it was rare to see a used Tesla ($NASDAQ:TSLA) available at CarMax ($NYSE:KMX), the world's largest seller of used cars. That was mainly because Tesla is a young brand, having only delivered cars since 2012.

But it appears that first-time Tesla owners are finally selling their cars, and that means that prices for used Teslas are slipping at CarMax for the very first time.

We track used car inventory at CarMax, and by averaging the cost of Teslas over time, a clear pattern is emerging as of this fall and winter: as the supply of used Teslas increased in October, the average price dropped.

If that sounds like simple supply and demand economics, you'd be right. But in this case, we're talking about an item that has historically been rare.

On October 1, 2018, the average price for a used Tesla at CarMax spiked to $87,000. But, if you look at the inventory numbers, you also see that on that very date, Carmax listed only one available Tesla. By November 12, 2018, Carmax had 12 Teslas for sale. The following day, the average price for Teslas dropped to $71,900.

As of December 5, 2018, the average price of a Tesla at Carmax settled to $60,400. In short, that's a drop of $26,600, or 31%. In one case, a 2015 Model S 85 with 44,000 miles can be had for just $49,998 — that same car retailed for over $81,000 just three years ago.

This isn't the first time we've seen a drop in used Tesla prices along with a rise in inventory. In September, we reported a steady decline in used Model S prices at both Carvana and CarMax. It appears that trend is sticking, and prices continue to drop.

Interestingly, CarMax doesn't list Teslas often on its public-facing website, but it does list them in its public facing API, which is were we pull our inventory data.