Bought an "unlocked" S21 from eBay, but it's actually locked

I recently bought an S21 from a seller on eBay that's described as being a top seller. It was listed as an open box item, so I pretty much summed it up as getting a steal. 

I screenshotted the original listing which mentions that the phone is fully unlocked and that any SIM card would work. Only after trying my own SIM card (and another one from my girlfriend who's on a different carrier), did I confirm that it's not unlocked at all. 

The phone immediately booted to a fresh home screen the first time. Then I noticed that there were AT&T apps. I thought nothing of it. But, after running into the SIM lock, that explains why. 

I've contacted the seller,  but it's been nearly a full day and I haven't heard back yet. I haven't used eBay's Money Back guarantee just yet,  nor have I disputed the charge with PayPal yet. 

I used AT&T 's online unlocking tool and it says that the IMEI shows that the phone hasn't been fully paid for yet. 

I'm not sure what to do at this point. Any suggestions?