Buying used speakers from a private seller for local pickup.

I would assumne at the very least, you would need a bill of sale and probably 

If the speakers were made anywhere besides the U.S. or Canada (or any other NAFTA country) you might be required to play an import duty. So it would be important to be able to have evidence of the place of manufacture. Of course, customs agents have very little to go on to establish a country of origin or a value if they do think you need to pay duty. It can be a frustrating process if they want to pursue it so have as  much documentation as you can, and be prepared to be creative in your answers.

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.