Trust

Nothing is guaranteed, but the one thing I always do is check the sellers feedback. I just won't buy (a high dollar item) from a new(ish) member or a member that doesn't have a fair amount of feedback or negative feedback. 
Unfortunately large speakers are the most prone to shipping damage, especially if they don't have the original boxes. 
Also, I know this is after the fact, but Paypal will be on your side if something goes wrong.

Great question as I’m soon to be listing a large expensive pair of speakers... I guess a nice FaceTime session or video of a specific request can verify condition and functionality.  But shipping such items is expensive and of higher risk. Best option is local pickup and pack up as if to be shipped.  Good luck and keep us posted. 

If the guy does not have many GREAT feedbacks on Audiogon do not buy from him as you will be sorry in the long run.Look for a speaker from a trusted member only.

You could also have a friend/ relative or forum member go and look at the speakers for you. 

If the seller has good feedback, then my biggest fear would be shipping damage.