Bad Imei if the owner did not pay off the phone?

Return it for a refund. If the seller refuses, file a claim with eBay. There is a good chance it's been reported lost or stolen or that the seller skipped out on their contract. (yes, it can be reported stolen if the box is unopened... it happens all the time, and is a common scam. The seller orders a phone, reports it stolen in the post, then sells it and gets a replacement through their carrier).

Do NOT wait several months. eBay has a limit on how long you have to file a claim.

Buy a phone from a legitimate source. If someone is selling an iPhone 8 for less than the full retail price, something is wrong.

TANSTAAFL

To add to Tim's excellent advice, if you paid less than list price there is almost certainly something wrong. They are in short supply, and prices outside of the official supply chain are higher than list, not lower.

I should also point out that it is trivially easy to shrink wrap an opened box and make it look like it was unopened.

I like the above response far better than the one I wrote.