OP - the offer you received is too low compared to national averages. Either this is an extreme outlier or something is wrong with the car.
Any offer you receive from the places you cited is based on national used car price averages. Before buying a Tesla, I tracked prices on all models for about 9 months. This site is a good one to start with, it's a good benchmark for the state of the market:
Used Cars for Sale Ranked by Best Deals & Price - iSeeCars.com
Find 4 million cars for sale all in one place. Every listing is analyzed and scored to help you get a great used vehicle at a great price from a great seller.
www.iseecars.com
Nationwide, the current average dealer price for a 2018 M3LR is around $35,000. The average number of miles is around 50k, When trading in, you should expect to receive around 80% of the dealer price, subject to adjustments around condition, mileage, and other factors related to the quality of the vehicle. Anything less than that and you need to shop around the trade in.
Another factor affecting price in inventory, if dealers are having trouble moving cars, they will offer you less for your trade in. There are 51 listings on iseecars and 800 on Autotrader (which overlap, iseecars is a subset of what you would see on autotrader.) That is a low inventory, I've seen as many as 2,800 on a M3LR within the last 6 months. The low inventory suggests this is a good time to be selling used Teslas.
Carmax / Carvana / other nationwide dealers offer convenience. They do NOT always provide the best offer on a used car. To maximize the price you get for your trade in, you should get estimates from local dealerships. This can often be done on their website, but you can also just call them and ask for a ballpark over the phone. They have access to the same data that's available on iseecars and will likely use a similar calculus to what I described.
Alternatively, you should consider a private listing. One thing to understand, the tax laws in your area may benefit dealerships. Where I live, there's a flat tax on new car sales. There's also a dealership exception, where a buyer is only taxed on the difference between the price of the new car minus the trade in. This can work for you, the strategy seems to be listing the car for 90% of the average price charged by local dealers. You come out ahead compared to a trade in, the buyer comes out ahead based on final price. While I haven't done this, I've heard it's even possible to finance a private sale so long as the car has passed a recent inspection. The paperwork most banks need - a Motor Vehicle Purchase Agreement - is available as a template online.
One other thing to remember, with a private listing, is that exposure matters. The best places to list are autotrader.com and ebay motors. Part of the reason for the later is the consumer protections it offers buyers, it's similar to what they offer on other purchases from the site. I was pricing a Challenger through these sites in October and saw listings throughout the US and Canada.
While I did reserve a MYLR last Friday, I did seriously consider a used vehicle instead. One piece of advice I've received from multiple dealers, Teslas have so little going on with them mechanically that miles don't matter. If someone is trying to argue you down based on miles, I would resist that and point out the vehicle only has 16 moving parts. That seems to be a proven sales tactic.
Good luck.
