Seems to be extremely variable.
The trade-in price for my 2016 SEAT Leon was pretty competitive. Within about £1500 of what I would have got privately at the very most, as I had some cosmetic damage from a numpty old bloke in a BMW that reversed into my rear quarter in a car park, and they weren't bothered about it at all, so likely under £1000 difference once that damage was taken into account. Definitely not worth the hassle of a private sale during a pandemic for that amount!