Paying taxes/duty on used goods purchased in Canada

Yes, it all depends on the shipper. Anything sent USPS should not exact a fee. I've bought three folding knives (on three occasions) from a Canadian site and the postage for each was around $3.00 and I got them within 5 days. No problem, no hassle. 

Something like your speakers can present a problem due to their size. You can't get around that. A few years back I sent an amp back to a Canadian site for a mod and was told to state that it was for repairs and never was assessed a fee of any sort, just the cost of the unit, size and weight. This was through FedEx. It went straight through with no hassles, both ways.

All the best,
Nonoise

Think about it, it being the North American Free Trade Zone.  Nothing free about it.

The question about where the speakers were made is the most important one. If they were made in England you will be paying import duty in addition to any shipping and brokerage fees. If they had been made in Canada there would be no duty because Canada is one of the members of NAFTA.

I bought a pair of Proac speakers, made in England, from a seller in British Columbia and drove them across the border to Seattle. At the U.S. customs office at the border I had to spend an hour waiting for a customs officer to go through thousands of product listings in a book that looked like the Manhattan phone directory. She eventually gave up and asked me what I’d paid for the speakers ($1,400) and she assigned a duty of something like $200, which was totally arbitrary, of course. So, there’s a certain amount of uncertainty in the process.

I'm not sure how you would accomplish it but getting an answer in advance from U.S. customs could avoid an unpleasant surprise.

I bought 2 used pieces of equipment from Canada several years ago, a phono pre-amp and a CD player. In both cases the seller lived near the border and drove the items into the US and shipped them. They said the border guards never checked. Of course that depends on the  check point. One seller said the US site was the nearest FedEx site and people did this all the time, especially for eBay sales. Might be harder with speakers.

How about the actual fee schedule (assumed current):

https://www.ups.com/content/us/en/shipping/cost/zones/customs/fees.html

If it's like importing into Canada, you will have to pay the brokerage plus a disbursement fee and a COD fee.

Duties - I don't know, but would assume at least 6%.  Taxes - depends on were you live.

Just assume the worst, and if the deal still makes sense, go for it.