Return it. Make sure you do so with package tracking though!
I bought an s4 40mm aluminum from Amazon Warehouse Deals last month, listed as used-very good condition. First one still had all packaging, but contained an activation locked watch. Contacted Amazon, arranged an exchange. Second one was again still in its protective wrap, with everything in original packaging, and paired fine. Both watches were still within the period where AppleCare+ could still be purchased—I think it’s likely the first buyers either decided to upgrade size, materials, or to the cellular model, since neither showed physical signs of use. Not a scratch or nick to be seen, and a significant difference in price, cheaper than buying used on Swappa.
The return itself was something of a nuisance. Prepaid UPS label from Amazon, UPS delivered back to them quickly...but they didn’t register the return. It took almost a month, and a couple of chats where I sent them the tracking number, before Customer Service finally said they couldn’t find it but I wouldn’t be charged. A week later, I finally got their confirmation of the return. 🙄