Tim, it's not always so obvious either
In short, yes, I'd require a seatrial.
I'd been had.
Sometimes urban myths are real, sometimes just myth.
When we sold our C25 in 1998, we had a couple and their two kids come on the sea trial. We'd done all the "homework" as suggested here: deposit, written agreement pending sea trial and survey, haulout, etc.
Had a lovely day on the water, I got to sail the boat "one last time" and they seemed to learn a lot and enjoyed it. The wife was so busy making new "rules" for the kids that it was starting to take all the fun out of the day, so I suggested two simple rules that we've always used to keep it simple: 1) One hand for you and one hand for the boat; 2) When the skipper says to do something, do it, first & immediately, then, if necessary, ask why; never the other way around.
Those two have served us well for over a quarter of a century, with an easy going approach to fun and enjoyment, as well as learning. She started using it as we turned for home, and all of a sudden the yelling, arguing and disagreements disappeared and even the kids started having fun.
Man, I thought, this boat is now sold.
NOT.
When it was hauled the next day, there were a few blobs on the painted lead keel. She mistakenly thought they were fiberglass blisters and literally ran screaming from the boat yard!!! I have this picture in my head like a cartoon character, of her running away, pursued by the surveyor brandishing his survey yelling, "But it's only paint on the keel, the boat is perfect, why are you running away!?!?"
The next week a fellow on our dock saw the for sale sign still there, called me and bought it without another haul simply based on the week old survey.
He still has the boat. On the same dock since we originally bought her in 1987.
And I never knew if the family was trolling or not.
That said, when we bought our C22 in 1983 we didn't bother with a sea trial. The boat was only a year and a half old, in perfect condition on a trailer. Older boats may require a different approach. But rarely is the swing keel a problem if the rest of the boat is well taken care of.
Kinda like this:
My experience has been that a close observation of the seller and those things surrounding them can often give a clue as to the seller's approach to care & maintenance in general. Rare is the person who lets their car stay dirty, with leaking oil, grating brakes, etc., while meticulously caring for and maintaining their diesel engine & fiberglass boat.