Can I trade a Apple Watch I bought on eBay?

If you are able to do all the steps outlined in this link, then you should have no issue trading it in --> What to do before you sell, give away, or trade in your Apple Watch, or buy one from someone else - Apple Support

Depends. Did this watch you bought on eBay come activation locked and perhaps you don't know how to use the watch? If you answered yes, then no, you cannot trade such a watch to Apple. The watch would have to be fully prepared to be traded and if you by chance don't know the Apple ID or Password used to set up this watch, its worthless.

On the other side, if this watch purchased on eBay is one you can remove activation lock from and prepare it properly to be traded in, it works and isn't damaged, then yes, you can trade it in.