Quoted from frolic:
if you bring it up for them, you'll be charged 5%. if they bring it up, they pay 13%.
Not so sure that's accurate any more - one of my buffalo friends brought a machine over for someone and wound up paying the 13%, plus it was much more of a pain for him that it would have been for the buyer. I'm grabbing one soon, and counting on just paying the 13%. I got hauled out of my car once for a deal that the border didn't believe because they found one on eBay for double what I paid. Thankfully eventually they believed me based on condition, but it sure wasn't fun.