I imported my boat last year. Here is a bit of my experience.
If it is a private sale(ie; ebay, craigslist etc.) it's a very good idea to bring along all dialogue between you and seller (emails) bring pictures of the boat etc.
The only hiccup I had was right off the bat. If you're bringing cash, more than $10,000 as I was, you must have paperwork from the Canadian guys before entering the states, basically Canada wants to know why this amount of money is being taken out of Canada, and the American's want to know why you are bringing this amount of money into the US. This is where I screwed up, I didn't have the paperwork from the Canadian guys. Long story short, the Americans took me inside, took my money and me in a back room, interogated me a little, this is where the dialog, pictures, emails etc. came in handy, counted my money and eventually (after I just about crapped my pants) gave me my money back and sent me on my way with a stern warning of what I should do next time!
Run the VIN# of the trailer with RIV before going down there to ensure you can bring the trailer back across. The trailer is what matters, they didn't even look at my boat.
Every state is different, and insurance can get tricky, if you are crossing more than one state, check to make sure you are insured. I bought binder insurance(locally) before I went down there just to be safe.
This website
http://boating.ncf.ca/buy-usa.html was very helpful and answered most of my questions.
Good Luck, you'll save a pile of money if you do your homework. I saved several thousand on an aluminum boat made right here in BC, re-branded and sold to the states for thousands less(kind of sad really)!!