ebaymotors.com, cars.com, autotrader.com, autotempest.com, vroom.com, craiglist.com, autobytel.com, truecar.com, and of course
Tesla for Sale
Personally, I have bought and sold successfully on ebaymotors.com including a Tesla Model S sale (P85) and purchase (P85+).
In fact all my cars but one (our 90D) have come from ebaymotors.com for the last six purchases. Tesla P85+, Passat, FJ Cruiser, Zero MC, Miata, VFR800 MC, all bought off ebaymotors.com. Just avoid the dealers and you will do fine and make sure you take alot of pictures - especially the screen the shows all the options your car has (air suspension, pano, subzero). I have been lurking and waiting for P100D's to hit the low $80K, but the biggest pet peeve are the guys that list all the standard options and then don't show what PAID options the car has.
I love ebaymotors because I bid on multiple cars at the price I want and usually in a couple of months I get it within the budget i want to pay.
The odds are you are better off handing the keys to Tesla on a 85D.
here are some recent sales on ebay;
2015 Tesla Model S 85D | eBay 2015 85D
Tesla Tesla in Cars & Trucks | eBay sold only reflects 2015
Tesla X in Cars & Trucks | eBay - sold only reflects 2016 and 2017
tesla s in Cars & Trucks | eBay - sold only reflects 2016 and 2017