Several issues at play here.
1. Getting a clear title from the seller.
2. Importing the boat into the US
3. Getting the boat retitled or documented in the US
4. Seller entering the US on the boat.
A couple of things I can comment on, a couple where I cannot.
1. No experience with foreign titles, deregistering, etc. From what Boatman61 says it sounds like the issue may vary from country to country within Europe
so you will probably need to give the specific country of registry to get a detailed answer.
2. I am in the import/export business and suggest you contact a customs
broker to handle this. These are companies that do nothing but manage the paperwork for importing things into the US, handle the duty, etc. Send me a PM and I can refer you to the company that I use.
3. With clear title, bill of sale, etc then this is a straightforward process for state registration
. If you want to do USCG documentation
I would check with their website or call to confirm what paperwork they require.
4. If the seller is coming to the US by yacht he/she will need to deal with the proper visas, etc.