From Canada to USA - buying used stereo from private sale

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.

I have done well from Canada, and, I had a true disaster.

think protection.

CREDIT CARD: use your best most protective credit card for the payment to paypal then to seller. you might need to change your preferred payment options temporarily to your card.

PAYPAL: only gives protection for GOODS, therefore you need an invoice from the seller.

If you simply send money, it is like sending money to friends or family, no coverage.

EBAY: sold PayPal, and if you give eBay your bank's information, then they send money to your bank, skipping PayPal fees

even if seller asks for you to pay his additional fees, I think it is worth it to get protection.

INSURANCE: the seller buys the insurance, and the seller must make any damage claim. The item will need to be returned to seller's freight company for evaluation. Improper packaging might be an issue, so, keep packking materials, pay attention, i.e. take photos as it comes out of the box.

VINTAGE: do not let seller put vintage on the invoice or description, as, after the fact, after selling seller insurance, UPS Canada said: "we don't insure vintage".

I agree, Canada is not trouble, my post about the tariff rules was only to give a simplified explanation of the process. I've bought and sold lots of items without problems to and from Canada through FedEx, UPS and Canada Post as well as driving them back through the border crossings when I lived in Seattle.