You definitely have the right idea about buying a US bike down there and riding it back up.
I think the most important part of this kind of deal is making sure that the previous owner is there in person at the point of the sale and is willing to ride it across the border and back with you so you can insure that it works. The worst case scenario would be that you guys get a gestapo aduana agent and both get slaps on the wrist for trying to pull a fast one, but I think there is a .0001% chance of that happening.
The second important part of this deal is checking the title transfer regulations between your state and the seller's state. If your state requires an inspection of the bike before a title can be issued then you better come up with some kind of solution or you will not be able get the bike across the US border. You also want to check to see whether your state requires you to be there in person to do a title transfer or if you can do it by mail.
I know that Xfiltrate will not agree with this part because it it kind of illegal, but I am only providing this information for riders who plan on eventually legally importing there bikes back to the USA or Canada.
Once you have checked the title transfer regulations between your state and the sellers state and made sure that you can do it without an inspection then you can proceed with buying the bike.
The only problem is that when the seller and you go to cross the border you will not have the correct paperwork to be able to get the bike into your name. The solution for this is to make a good color copy photoshop of the sellers title, registration, and temporary import permit. The only change that you need to make is swapping out the seller's name for your name. Then you will be able to successfully get a temporary import permit in your name and get the bike stamped in your passport. Now you will be able to travel all the way until the US border with these documents. I drove from the U.S. to Argentina without the title of my bike, I only had a good color copy of it.
The next step is Fedex'ing the seller's original title to a friend back home who can take care of the paperwork for you. Hopefully, your state does not require that you be there in person to do a title transfer, but if it does you need to come up with a solution for that.
Once you have the real title and registration in your name then have the friend Fedex it to a contact who is a few countries North of you. That way, your documents will arrive before you arrive and you will not be waiting on them. I have to emphasize the importance of using FedEx even if it costs you $100 each way it is worth every penny of the price. If anything happened to your title then you would not be able to get into the US.
btw: My Australian friend sold his nice KLR to a Polish guy in Argentina and they crossed the Chilean border together to do the deal. The Polish guy made it all the way to Mexico with a photoshopped Canadian Visa, title, registration, and insurance. lol
I do not recommend doing what the Polish guy did though because it would be highly illegal to do something like that in Western society.