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. 

Make sure the boxes are strapped to pallets and shipped by freight. 

Have the seller take pictures of each stage of packing prior to shipment. “Inadequate packaging” is one of their favorite allegations in denying insurance claims.  

See if they have a UPS store in their country. If they do, I have the seller contact them and pay them to pack them up. Then ship if possible

Don't skimp on packaging. It cost me a lot to ship Quads on a pallet but it was worth it.

No, do not let UPS stores pack large heavy items, they do not know how to properly. helomech & noromance are correct, they need to be boxed (ideally in the original boxes & packing) strapped to pallets and shipped by freight.

Just like above, do not let UPS store pack large items. UPS store is used to packing everyday smaller items. I shipped an 88 key midi keyboard thru the UPS store and the item was basically destroyed. I had to fight UPS insurance because they said it was poorly packed. I had to pull receipts, etc from the store. UPS said, the stores are independently franchised and not tied directly to the company. I had to fight the store in which they claimed they are not responsible for damage. I ended getting 40% of the insurance. I then used that 40% to purchase a refurbished unit from the keyboard company and had it shipped to the buyer who was happy.

This place does "freight"

https://www.pakmail.com/