I suspect it's a $4,000 trailer and a free horse. Said horse is definitely not broke, maybe bad with farriers (based off hoof condition---they seem a bit underrun, unless the shavings are obscuring actual hoof angle,) and would most likely be hard to sell in a hurry if they hadn't thrown in a deal like a trailer. Unbroke, dark-coloured stock horses abound in the "used horse" market, at least around here, and those actually wanting to go to the effort of training their own horse or finding a trainer for it (and paying the associated $$$) are at least potentially likely to be going to a breeder instead where they can be certain of their background. She seems to be built at least not badly---though her shoulder seems a bit steep, and she might be built a bit downhill---but she's not squared up and I'm not the best at horse conformation in the first place.
Plus, I see rust on the trailer, so it's not new. Maybe the person just needs the horse out now, and has a trailer they're not using any longer; or the trailer is rusted out, the horse is unmanageable and/or lame and they're trying to get rid of both for some cash while they still can. "Super healthy" isn't very specific, and most people selling horses with good feet tend to specify that. Those would be best-case and worst-case scenarios, respectively.
