Its the moment you step out after signing the ownership transfer, vehicle will start depreciate and starting around 80 ~85 % of the MSRP value regardless of the mileage driven. (of course except some cases where vehicle demand is so high compared to the actual supply then it gets tagged with premium)
Dealer usually takes about 15 % of the margin on used car trade-in and sale. That will be lower for their own brand vehicle that can be easily certified (with valid warranty). If you bring other brand vehicles then, the value will be even worth than the market price.
I personally never recommend anyone to do anything with dealer except;
when you are buying certain "dealer only offered" vehicle/ and getting your service under warranty.
Check;