What do I need for customs if purchased in Canada & ship to US?

When the seller buys the postage he completes the form. You shouldn't have to do a thing. The seller states what's in the box and what it's worth. Not a big deal. Cheers,
Spencer

I had a bad experience using UPS to ship from Canada to the US. They billed me for customs, and imposed a brokerage service charge, for an item that should not have been subject to customs. It was for a Triplanar tonearm that I already owned. I sent it to the builder of my turntable so that he could drill the holes for mounting of the arm. UPS billed me for the return shipping of the tonearm, but not for the turntable. Weird. Since then, I insist on using Canada Post for items shipped from Canada. No problems with them.

Thank you to all!   This is a big help.   I intend on requesting that anything I buy be shipped Canadian Post or FedX.
Thanks again.

I bought a used B & W subwoofer from Canada....had it shipped via UPS to TX.  Unknown to both me and the seller, the import duties are assigned based on the country of manufacture.  Since the sub was in the original box and B & W products are made in China, I got hit with something like 5% tax...including some extra fees they helped themselves to it was about $150.  I wasn't that thrilled.  You can go to this website and fill in the parameters to see what kind of taxes you are looking at. https://www.dutycalculator.com

Any customs forms will be completed by the sender. If the item is light enough it should be sent through Canada Post. More than likely the item will enter USA with no problem. If the item is made in Canada there should be no duty, if not you COULD be accessed duty but probably not. Country of origin is filled in on the customs form.
I frequently buy and sell overseas and have paid duty and fees only once courtesy of UPS.