From Canada to USA - buying used stereo from private sale

As audio2design said, what matters is where the gear was made. Last I dealt with it, gear built in the countries that are partners with the U.S. in the NAFTA free trade agreement, Mexico and Canada, is duty free. So a pair of Paradigm speakers built in Canada should be free of duty. But a pair of ProAc speakers, bought from a seller in Canada but manufactured in England, would incur a duty, determined by the value and the existing trade agreement between the U.S. and England. This is complicated by whether the authority at the port of entry bothers to check and is able to figure out what they’re dealing with. That’s often not the case. So the answer to your questions is, it depends.

Canada is not trouble.

I’ve literally shipped thousands of items into the USA.

UPS and FED-EX are the ’easier’ to deal with, regarding the paperwork filing and filling and the guarantee of delivery...

Whereas Canada Post to USPS and then delivery to you, is more tricky and fraught with potential hassles and problems, due to learning to navigate the issues and paperwork. But also less likely to get dinged for the duties and taxes. Depends on the given item and whatnot.

Smart money is on UPS and FED-EX as a shipping method.

Especially if one is unfamiliar with the shipping scenarios and their possible good and bad bits. Which your OP flat out states, thus, it’s UPS or FED-EX for you.

I’m fine with any of them or all, as I’ve got the battle scars that taught me how it all works and shifts about.

Correct regarding no charges for used gear. Also correct about country of origin. 
I purchased Audio Note dac from Canada and paid tariff based on UK rates.

Agree to go with UPS or FedEx, but be prepared to pay COD for tariff. If there is a tariff on your item make sure to have cash on hand, credit card was a  PITA.