Sometimes it works out fine.
I recently responded to a craiglist add for pantera wheels and tires from Michigan. Seller ansered back. I never haggled on his price, but did ask for a shipping cost. We agreed on the total cost and then the hassle started. I explained that I was not really comfortable sending a guaranteed payment for goods I had not received or even seen,and I assumed he was not comfortable sending me an expensive set of wheels without getting paid first. Pretty reasonable for both of us. We e-mailed back and forth for almost two weeks trying to work this out. UPS will not ship COD from the US to Canada, so that was not an option. It was a lot of hassle for both of us, probably more so for the seller than me. After all the e-mails, I think we just got a sense about the other. We never actually agreed to it, but he ended up shipping the wheels and tires before my money arrived and I ended up sending the money without any holds before the wheels arrived. I guess we just decided to trust the other. A little trust and faith in your common man can go a long way.
My money arrived in his bank the day before the wheels arrived at my door. Exactly as he had describe them. No surprises.
Unfortunately, it obviously doesn't work out this well every time, but hopefully almost every time.
Keep the faith. I think it is important for all of us on both sides to be able to buy and sell across the border without the fear of being ripped off.