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.