I was on the phone with Apple for over an hour yesterday getting this worked out. They would not allow me to enter the purchase date without an original receipt. When I explained that I had no access to the original receipt, they said to contact the original owner of the watch to get it and they would not tell me the purchase date or warranty status because "privacy is embedded in Apple's DNA." I told them I am now the owner of the watch and it's activated on my account, so they are not violating anything by telling when my warranty expires. The conversation kept going around in circles.
Finally, I insisted on talking to a level 3 supervisor and that lady also would not give me warranty status or a purchase date to enter. She did finally tell me that the watch originally shipped in November and she could update the purchase date to within 3 months of that. So, I now have warranty coverage through February, 2022.
The original owner used UPS to pack and ship the watch. When it arrived, there was something in the packing material (like glue) that got all over the surface of the Digital Crown. It took me a while to carefully get it off without damaging the watch, but I finally did. Because of the activation lock snafu (he forgot to turn it off before shipping, resulting in a LOT of back and forth) and the UPS incident with the glue, he knocked another $100 off. He was very apologetic/embarrassed throughout the process, and a really nice guy.
Bottom line: I now have a $650 Series 5 White Ceramic on my wrist with original warranty through 2022!