Food for thought. I did a lot of research here on the TMC forum, by going in to a Tesla Service Center to sit in and test drive the MS. Ok, I went by and sat in it a few times, then while traveling for work I sat in the Model 3 for about an hour. Tried all the seats, features, frunk, trunk, touch screen...then went and sat in an S immediately in the same show room. I was spoiled, the only one in there on a weekday (Aventura Mall, Miami) and did the same thing. I came to the conclusion that I really liked the S.
1. Ask the current owner to have Tesla print all of the service records. This is one time where having more things replaced on earlier cars is better. Drive units, MCUs, door handles and on early cars, even batteries, are good things to have been changed out.
2. I'd be cautious buying a low mileage Model S unless it comes with a warranty. Higher mileage cars that have been serviced when required are going to be more reliable in the long run because they've already been tested. I went with a November build 2014 Model S 85 with 115,000 miles on it. There is still more than 4.5 years left on the battery and power train warranty. The higher miles helped get the price in a sweet spot for me ($42.5k).
3. Buy a car with features you actually need and will use. I really wanted to minimize future issues from popping up. I went with coil suspension, body color roof, textile seats, subzero package, 19" wheels, AP1 and the 85kWh battery. It was perfect for me. The body color didn't matter to me, but gray is a color I can live with. I wanted to use it for long road trips so AP1 and the 85kWh battery were important to me. The 19" wheels ensured I could buy tires that were reasonably priced and the set I just put on is warranted by the manufacturer (Bridgestone Serenity Plus Low Rolling Resistance) for 50k miles. Tire rotations are free for the life of the tires from Firestone. Other places do the same deal. Not having the panoramic roof ensured there would be no leaks or roof glass repairs. The textile seats are surprisingly comfortable and still have leather like trim in the right places to make them easy to get into. Coil suspension is much less expensive to maintain over the life of the car and rides quite nicely. I like the connected feel. The 85 non-performance car has outstanding performance in "Standard" mode. Most of the time I find my wife and I use "Chill" mode to save the tires and extend the range. Still, "Standard" mode is faster than the ZO6 Corvette I used to have when I want to play a little.
3a. Rear wheel drive is very capable, even on snow and ice as long as you don't have a steep driveway or plan on driving over unplowed roads. Tires are important, do some research. I listed what I chose above after lots of reading, then using the LRR tires on another car. Quiet tires that are LRR enhance the driving experience. Getting as many miles as possible for your money on tires is a personal decision, some don't care.
4. Since the car was higher miles and did not have some of the features a lot of other potential Tesla buyers usually go for, it was on the market for a little while. This also helped the price come down a bit as sellers usually get more realistic with the asking price as time goes on and it is an easier buying process when the negotiation is right around the kbb.com private sale price or below.
5. My parking situation can handle a car with the physical dimensions of the Model S. Some people are not as fortunate. Something to check. Parking is a challenge for a lot of people. Size of spot and the ability to charge at home are important to how much you will enjoy driving your Model S.
6. The used private party Model S market here (U.S.) in the Spring of 2018 seems to be $35k for an S60, $45k for an S85. The price varies based on the features included, usually in the upward direction by about $5k. Expect to pay $10k more if you want all the bells whistles, plus AP1. I live in Western Pennsylvania. I was looking at cars in California, Texas and Idaho. Being in LA you will likely have five or six cars easily within a day's drive you could look at if you decide to shop here on TMC or any of the other private party car buying sites.