Tesla really does have a strong influence on used Tesla pricing. There are 500 used model Ses showing up on Hank's site in the U.S.A. and 400 on all of Autotrader. Sure, not everything is on Autotrader, but Tesla doesn't post everything they have on their site either. Minimally, I think it is fair to say that fully HALF of all used Teslas for sale are being sold by Tesla themselves. When you combine this with the lack of understanding about the car by most resellers, the unknowns about repairs, and lack of warranties from third party resellers, you can see where Tesla has the ability to control price.
As an aside, I don't think any of this translates to great trade-in values for current owners. Indeed, I often wonder (perhaps cringe at) what former owners got for their trades when I see used Teslas for sale at local dealers.