GSP is great for US sellers, but also provides additional seller/buyer protections. Yes, its more expensive for buyers, but they get insurance and better tracking. Never had any issues, used it many times over the years since they created it. Repackaging must be a non-US practice - never heard of it happening. Or if it does, the US team does a better job. eBay carries all the risk after its received in Kentucky, so if they transfer it to crappy packaging which causes it to get damaged, they have to reimburse the buyer. Seller is 100% protected.
Canada is a customs roulette country. Technically anything >$20 is taxable, but I've never seen any buyer <$100 get taxed, and over >$100 happens rarely if shipping USPS. If you ship UPS or FedEx, they have it built into their computer systems so its more likely to get charged. GSP does not use USPS for that leg (that would defeat the purpose of GSP), so that's why Canadian buyers get hit with the customs fee on GSP. Since CBSA fails (for whatever reason) to enforce the rules on USPS parcels, there is a perception that direct shipping is much cheaper. It's only cheaper if direct-shipping buyers keep dodging the GST bullet.