Boxes

I prefer to buy from sellers who have the original boxes. Buying used equipment is always a gamble and having the original shipping container eliminates at least one of many concerns for me. Others may or may not agree, but I'm only concerned with my purchase comfort.

Flatten them and stack the foam innards to save space, and yes having original boxes is a selling point and gives the shippers another chance to smash the crap out of them.

other factor here is that quite often, in my experience, shippers like ups or fedex will deny customer damage claims if something other than manufacturer's packaging is used for shipping expensive precision items like hifi gear...

I had a claim denied by UPS using manufacturers boxes with all proper manufacturers packing. They dented both mono block amps. Doesn't matter how well it's packed, the default is always to deny, then see where that goes. 

One thing is for sure, they don't like heavy packages.