I don't think you can really insure (by that I mean actually collect) a resale value. Don't insurance companies only cover Blue Book (or less) amounts?
I have this impression due to a recent experience of having a 1973 Datsun truck stolen and then recovered, although damaged ("totaled"). Insurance paid only half of the amount that you see these trucks selling for. We get similar offerings for it regularly.
We recently bought a Scamp
and when I added it to our policy I asked how they would determine it's value if there were a claim. I was told that it would be determined by a Blue Book like listing, and have no relation to the amount we paid for it. Also, they figure the insurance premium by how much one reports paying for the trailer, but that has no effect on what they'll pay out. I asked specifically about this. I researched only 21st Century Insurance.