I've also noticed in the past that USPS and Canada Post do not always share their tracking numbers with each other, which makes international tracking numbers basically useless.
Also, bear in mind that Canadian Customs (or any other country's customs office) has the right to open and inspect the instrument before forwarding it to you. In the case of an Ashbory I purchased in January, I actually had a useful tracking number. It noticed that Customs held it for almost a full 2 weeks before releasing it to Canada Post for delivery to me.
I understand your purchase was in August, so I can't imagine Customs holding it this long. At the very least, they would have sent it back if there was a problem. Maybe follow the other TBer's advice & consult eBay/PayPal. If the bass eventually shows up, then repay the seller in good faith.
Another consideration, which I really hope did not happen to you, and I wouldn't wish upon my worst enemy... An unfortunate story happened to my family many years ago around Christmas time. My parents sent gifts to us with a tracking number but did not request a signature for delivery. The package seemed to be unusually late arriving, so we checked the tracking number & it was labelled as successful delivery the prior week but we hadn't received anything.
if no one's home, at the discretion of the postal carrier, they may leave the package somewhere on your doorstep. They call it a "Safe Drop." The person(s) must have watched the postal carrier or maybe they simply snooped around our door. Either way, someone stole our package. When my parents called Canada Post, they suggested to always request a signature. Also, you may put a note near your mailbox stating "No Safe Drops at this Address". That way, they'll leave a parcel card & drop it off at your pickup location.