What you are facing here is an example of two very different approaches taken by the taxing and revenue collecting powers of the two countries, Canada and the USA.
I'm Canadian. On at least two occasions I have hauled antique canoes worth several thousand dollars from Canada to US destinations as a favour to fellow WCHA enthusiasts. I declare them to US Customs at the border, produce a bill of sale with the purchaser's US address, and am told "have a nice trip; enjoy your stay." One time, I attended an Ontario country auction as an agent for a major US antique dealer. I bought goods worth well over $10K on his instructions. I boxed them up, loaded my vehicle and approached the border. Couldn't have gone more smoothly. I declared what I had - no guns, no alcohol, no tobacco; just old stuff. Advised them I was headed to the nearest UPS outlet for onward shipping. Again, welcome; no one even bothered to open a single box. American friends of mine often tell me they have no problems bringing back antiques bought during Canadian holiday visits. Seems to be very little problem when stuff is destined to a US buyer or resident.
It can be a bit more difficult when I head into the US to attend antique shows or sporting goods shows. If I take things with me, it seems to help matters if they are intended mainly for 'display' purposes rather than sale. One or two friends have been turned back for trying to take things into the US which are intended for sale at shows. The reason is that US Customs then deem that the person is intending to work (sell things) to earn an income, thus needing a 'green card'. They are more lenient or understanding if the purpose is display with only the occasional possibility of a 'trade' or swap taking place.
Coming back into Canada from a US visit is another matter. Canadians are allowed certain 'exemptions' for goods bought while away. Zero for under 48 hours, a few hundred $$ for over 48 hours; more if you 're away for a week, etc. As Andre has said, there is no longer any 'duty' because of the "Free Trade Agreement". However, Canadian tax and revenue authorities devised a method of making a 'tax grab' anyway. Even if you bought something thousands of miles away, even three countries away, your purchase is deemed to take place as you step back into Canada. You are thereupon charged the federal "Goods and Services Tax" (currently 5%) along with the applicable Provincial Sales Tax (currently 8% in Ontario). So, show up at the Thousand Islands Bridge or Peace Bridge, and it will cost you 13% on top of the declared purchase value (very helpful to have adequate receipts). Sometimes the agents are busy, the items are small or look to be rather valueless, and you may get a break and be passed through. Better to be up front, honest, pay a few bucks than be hassled, have your car torn apart because the Border Officer (who is just doing his/her job) felt you intentionally were deceitful and got upset. Get them really angry and additional penalties and criminal charges can be imposed.
If I were an American hauling a canoe into Canada, and if asked at the border why I had it (unlikely in the first place because we almost expect everyone should have a canoe to begin with), I might be inclined to suggest I was simply hoping to do some vacation paddling nearby. The fact that you then forget to load it for your return journey is your fault (absent-mindedness); the fact that you had begun to doubt its sea-worthiness and simply abandoned it (preferably in your friend's yard so as not to litter the environment) rather than tote it home, is a 'judgment call'. Why waste money on gas for something you no longer care to have. It is unlikely that anyone will even notice.
