Something to consider if private sale....
You register the boat, not the outboard and therefore pay sales tax on the boat when you register it. If I was buying a boat tomorrow, I'd buy it in 3 separate transactions and get a bill of sale for each:
1. Hull
2. Outboard
3. Trailer(do you have to register the trailer in TN? I'm not sure)
When I registered it, I would only give the bill of sale for the hull(boat) unless I needed to register the trailer as well.
This is common practice in FL for private sales. Doesn't work at a dealership as they collect sales tax at time of purchase on the whole package.