Stolen property

Some thoughts:
If they don't have the box that's a worry, but not a deal breaker. Ask about associated equipment, cables etc, and how long they've owned them and how much they paid for them.

Know who you are dealing with: email, phone number, physical address

And use PayPay but NOT f&f: pay the stupid 3% for the peace of mind.

@jastralfu I thought your comment was spot on. 

On the subject: doesn't sound like it should be your concern. Imagine if everyone on c2c sales were asking this question. On every listing on craigslist you would accuse people to be thieves.

Wouldn't that be crazy?

You should probably practice up on your interrogation techniques and only buy local where you can read body language and voice intonation. 

reminds me another funny aspect:

"how will you buy it?

"cash?"

"is the money yours?"

"?"

"I mean you could have just robbed someone on the subway?"

I would suggest that just the fact that it crossed you mind it probably is not a good deal.  Every dealing I've had there was not a red flag (let alone a pink one).

There is literally tons of equipment for sale...only deal with reputable dealers with a track record.

Just my 2 pennies.

Regards,

barts