Stolen property

I don’t think you can, at least not easily. Here’s a few things to consider:

1. The price is too good to be true.
2. The seller is claiming to be ‘selling on behalf of a friend’.
3. The seller’s description indicates a lack of knowledge about the product.
4. You do a search and find the same description and/or photos used in an older listing by a different seller and there’s no obvious connection between the two.
5. You frequent forums and there’s a post by someone who has had their gear stolen.

A while back I was contacted by a person claiming to have legally purchased the contents of an abandoned storage unit, which happened to include Wilson Audio speakers and dCS digital gear, amongst others. He seemed fairly knowledgeable and his asking price for the items was consistently around Blue Book less 25%, so he appeared to have done research, if nothing else.

He seemed legitimate but it didn’t sit well so I passed on the deal. Some weeks later I saw where he had placed ads for the gear and someone had flagged them as stolen goods. I felt a little guilty for not being proactive in investigating him more thoroughly but also relieved at not having been scammed.

I thought speakers were supposed to be theft proof by being so heavy. I didn't realize there was a black market for stolen audio equipment. Although I am very concerned that I will wake up and find all the tubes missing from my gear these are much easier to steal.

And of course the cables could be stolen too very easily. 

Tubes and cables would be my choice since they are very difficult to prove whether they're stolen or not.

I’ve listed items that I know little about for friends before.  I’ve also had my descriptions of items I listed pasted onto listings by other people.  Sellers that see a good description feel free to use it.  Lack of knowledge of an item could indicate that an item was purchased for resale.

The best bet is to talk to the seller by phone before purchasing.

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?