Activated AT&TGalaxy Watch 5 Pro LTE number share on AT&T in store, less than an hour later got blacklisted text and phone blocked

Hello there, I purchased an AT&T Galaxy Watch 5 Pro LTE Unlocked from a very reputable Ebay seller to make sure nothing like this would happen. I went in AT&T they scanned the imei,set up the watch with number share with my phone,no red flags,all seemed good. I got home and after a little bit got 2 texts on the watch saying it was blacklisted which was quite a shock but it also kicked my S22 Ultra off the network which I didn't even know until my wife contacted me through a computer. The whole ordeal from start to finish getting my phone back working was around 5 hrs,online chat,on the phone and 2 different trips to AT&T stores.. Firstly I didn't buy a watch from an individual on purpose because I didn't want to take the chance that something like this could happen. Secondly how was this not caught immediately when they scanned in the imei to set it up on my account. Thirdly I told the vendor I want it replaced but they said they don't have any right now which is completely unacceptable to me. What are my options here?