Buying used speakers from a private seller for local pickup.

Well, they’re old and used Tannoy’s. I guess they’ll get a duty charge unless stereo gear is exempt.  What kind of documentation do you feel would be necessary?

Market value documentation with photos of the very same speakers to verify. Look up the actual percent or percent range charged by very specific category from the U.S. Customs online. Your knowledge of the categories will help 'guide' the agent assessing. 

My comments are based on only one experience, driving some built-in-England ProAc speakers from Vancouver Island to Seattle. No way to sneak them in since they were quite visible in the back of my car. What followed was probably half an hour of having an agent look through volumes that looked like the Manhattan phone book trying to identify the speakers and determine the duty. Somewhat arbitrarily we finally settled on $50 and I was on my way. Just a bill of sale might be all that's required, or anything else you can find to establish the age and value.