Stolen property

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?

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