How do I tell if a Radar Detector is stolen?

I’m running a small used book business.
An element of that business is an Ebay store.
I have a regular supplier who normally brings me a solid 10 valuable books per month; I pay him wholesale and then sell them at the online retail price.
I know where he gets those.
It’s a good arrangement, but it appears he’s got a radar detector now that he wants me to sell on Ebay.
I won’t touch it without having a good idea how he got it, but it made me think…
I have no idea how I would protect myself from selling stolen electronics if I was in the consignment business.
Can anyone tell me how you make sure electronics are not stolen in the absence of a retail or wholesale receipt for said electronics?

Any legitimate electronics items will include at a minimum the instructions, some or all of the packaging and everything that came with the item new. I know a couple people of questionable character and they occasionally bring me stuff to sell on eBay for them. The above is my requirements for anything electrical, especially auto accessories.

That makes little sense to me.

eBay is really two marketplaces: One for used goods, sort of a global garage sale / tag sale / flea market / <insert regional term here>.

The other is low overhead retail of new goods.

IANA eBay expert, but for used goods I’d expect zero of those accessory items to still exist. Personally I’ve historically been pretty meticulous about keeping instruction manuals even though warantee cards, packaging, etc go in the trash within minutes. nowadays Ipitch the increasinly useless manuals, confident I can find them online if needed. But I’m a lot more organized about that stuff than 95% of the people I know.

The typical consumer pitches all that stuff once they get the item to work the first time.

So if the OP’s used book source is selling a used radar detector, I would NOT consider lack of packaging, etc to be a red flag; rather the opposite.

Allegedly new, bought last week by the used book guy who didn’t need it but it was on sale and he couldn;t resists and now he doesn’t have the box? Now that I’d be suspicious about.