I have yet to truely understand shy so many sellers limit there eBay ads to US only. I'm in Canada, right on the same continent, and most of the US only sellers I contact refuse to ship outside the US. Is it that much more difficult to ship to Canada? Or is it just that every country outside of the US is renowned for trying to fleece every US seller so best stick to home! I would have thought that sellers would want to widen their sales area as much as possible to maximise the final bid.
In a perfect world, it would be equally easy to ship to Canada as it is within the US. But it isn't a perfect world.
In additional to all of the reasons mentioned, I've generally stopped selling outside the US because I'm tired of hearing the whining about why shipping takes so long and why it's so expensive. It's not worth it for me to spend the time and frustration replying to those messages knowing that they've never actually bought the items after I've put in the time to respond to inquiries. Selling to people in the US on eBay is enough trouble as it is, why would I add to the frustration?
Also, USPS will pick up packages for you for free, so you never need to drive to a PO or mailbox again to drop off packages. Just look into Carrier Pickup:
When I tried carrier pickup the first time, the packages sat there for 2 days until I gave up and took them to the post office myself. The carrier delivered mail both days and stepped right over the packages. The second time, one package was returned because the post office said the postage was wrong (which it wasn't) and I had to repay to ship the item because the original postage was "spent" when the package was picked up and returned to me.
I didn't give "Carrier Pickup" a third opportunity to screw me over.