I have a possible buyer from Canada. He was asking what the applicable taxes would be since I live in the States. Anybody know or have experience with this?

I know of a few people who have bought boats from the US and brought them to Canada. It is pretty easy to be honest. He will need the bill of sale and ownership papers. When he gets to the border he will have to pay the taxes then. It will depend on which province he crosses into.

I buy like 25 to 50 used boats a year on your side of the border. mmillward26 is right. As a seller you just sell it for the amount agreed. the buyer gets to pay all the fees at the border... so it is his problem not the seller problem

Does anyone know how is it in the reverse situation???---such as an American buying a boat in Canada and bringing it across the border into America....??? Is it a pain in the arss or what?

Should be much the same, no big deal. You will be exempt from paying CAN taxes since it is for export. I expect you will pay tax in the State you register it in. Just make sure you have you documentation in order at the border with proper Bill of Sale.
The DoubleR guys send boats to the US all the time and can guide you. Ask for Peter.
Bob