This does not sound like a major concern. And with only 18 hours in 3 years, you should expect a couple more minor problems to crop up. That's barely enough hours to break in the engine, nevermind to keep the gremlins away.
I would pay special attention to how the boat was stored. If it was left outside in the sun and rain, that takes it's toll on the interior (vinyl, carpet, etc), and even on the hull, engine and wiring. It may look like new when cleaned up, but 3 years of sun exposure can cause a lot of age to the expensive upholstery, and a little less rapidly to the gelcoat. However, if it was stored indoors or covered, then that is much better. My boat is 16 years old and probably only has a few months of sun exposure...it has always been stored indoors, in a boat house, and/or covered.
But if you like the boat and the price, then go for it. There is really not that much that could be wrong with a 3 year old boat with 18 hours that you cannot see with a discerning eye. You could take a more experienced boater friend with you on the next sea trial (one that has experience with Inboard/Outboards). Regarding a 2nd sea trial...if I were the buyer I would want to make sure I'm not just taking a freeloader out for boat rides...you could offer a non-refundable deposit of $100 or something to make it worth his while.
Ray
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"Knot Easy" 2000 Horizon 240 Volvo 5.7GS /SX
tow: 2017 Honda PILOT EXL-AWD
prev. boats:
'87 Chaparral 198CXL 4.3 OMC Cobra
'69 Jetstar 16ft Ski Boat, 115hp Yamaha
'68 Aluminum Jon Boat, 3hp Sears
'64 Water Wings