From Canada to USA - buying used stereo from private sale

I bought approx $1000 item from Canada-based vendor. No customs issues.

Used does not matter, country of manufacturing sets the tariff (if any). Usually low.  Get close on the tariff code.  You can usually do a basic web search and get something accurate. Taxes will need to be paid going into Canada on sale amount. That is buyers responsibility to pay. You can also have them responsible for duties and taxes. Just a check box on FedEx form. 

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.