I purchased an iPad last year and would likely have experienced a similar issue had I not spotted it.
In my case, the device was listed as in great condition and hardly used.
When it arrived, the iPad was in sleep mode rather than being powered off. After pressing the sleep/wake button, the following screen was displayed with a message to ‘Finish Setting Up iPad’-
View attachment 2289886
My first thought was to connect the iPad to my Mac and check the battery cycle count. It was essentially brand new.
Despite that, I decided to erase all content and settings to be on the safe side. While going through that procedure, I came across the following remote management screens-
View attachment 2289887
View attachment 2289888
Presumably if I'd completed the original setup process, I wouldn't have known about the remote management.
I’d already been suspicious because the seller had listed two other iPad Pros on the same weekend that I bought mine. The prices were also very good.
At first, I wondered if there was only one iPad Pro, but the serial numbers shown on the photographs checked out (at least two of the devices were registered on the same date). They’d also sold another iPad Pro a couple of weeks earlier, although no feedback had been left for that.
The seller had excellent feedback and there was recent activity, so no reason to think that their account had been hacked. I decided to purchase making sure that I was covered by eBay’s money-back guarantee.
Because of my suspicions, I decided to contact the institution shown on the remote management screen rather than the seller. I asked them whether it was a legitimate purchase. It turned out to be stolen.
I think the OP should try contacting the seller first, assuming they're still registered with eBay. If not, then I would still contact eBay as it's possible that other people have had issues with this seller. Tip - to actually speak to someone at eBay, go through the help procedure as if you're reporting a hacked account.
I doubt that Apple will be able to do anything because the OP doesn't have proof of purchase from either them or another retailer. It's possible that they will be able to identify whether the device has been marked as stolen.
If the OP suspects that the device is stolen, then they should contact the Police.
Given the timescale, I suspect the device was loaned to somebody prior to being sold on eBay, and because it has not been returned, the lender has deactivated it.