Regarding the OP, most of the time I just trade the old one in and recycle to the latest version each year. I've also sold them over EBay and to a 3rd party.
Thing is, resale on these things in the toilet. There's too many of them and the annual upgrades just aren't offering anything worth upgrading to, so really, a 6 isn't much different from an 8 for most people.
If Apple didn't offer the Ultra this year I'd be wearing my Garmin Epix 2. It's a good watch, better than an AW8, but the Ultra closed the gap and I finally found an affordable provider for cell service, so an Ultra it is.