I second calling Central Marine down in Florida. Ask for Karl Neiger. He sold me a pair of F300s last year, and I could not have been happier with the price or the level of service he provided. I live in NC, and he had them shipped up for right around $500 in freight costs. This will also allow you to shop around to get the best deal on the install. Let me tell you, the quotes I got to have them hung varied WIDELY. I think the highest quote I got - just for labor - was $9,000. In the end, after shopping around, I paid more like $3,000. If you currently have mechanical controls and buy mechanical engines, it also saves money as the installation is a lot simpler without having to re-rig new connections for the digital engine and control. Obviously, you also wouldn't need new gauges, controls, rigging, etc. I went from mechanical to digital, and I think most of the labor hours spent was in running the new connections.
In regards to selling, I sold the 250s that I took off the boat, and it was not that hard. Craigslist, THT, FB Marketplace, etc. will all work. In the end though, I sold them to a friend of my mechanic. It seemed to me that people in the boat business were the most serious potential buyers. If there are some boat yards and service shops around that will let you put a flier up, it would be worth your time.
Good luck!