As you know, fraud is real and the thieves are smart these days. If you are selling a bad drone, you're not going to provide a s/n that can be "looked up" and found to be bad. Therefore, lookout for the fake s/n provided to you which is likely social engineered from someone else on the internet or easily found elsewhere and it's clean. Same for stolen drones.
Just remember, the s/n you are provided by the seller for an item that you have neither won nor placed a bid on, you're not guaranteed to get that particular drone and the thieves know this. You get the green light from DJI, your drone arrives and it's a different locked drone s/n. If the seller is willing to list the s/n or provide it thru secure messaging to a legit buyer, that would be better (which I believe ultimately is the suggestion here). Nothing you can't overcome but just be aware, if you don't have the product in your hand, no guarantees. I know for sure I wouldn't give my s/n to just anyone who asks on the internet because they were browsing; only to the winner.