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The lister is trying to make specific use of terminology ("not a restoration" vs, a rennovation). Regardless of what they call it, this trailer appears to be almost completely rebuilt. If they did it right, then I would say that $12k is a very fair price.
They say the AC was never hooked up, but if it is just a matter of making an electrical connection, that is trivial. Even if you have to run a wire, it is not rocket science to do so.
I don't see any mention of the furnace at all. They also mention that the fresh water tank isn't "hooked up." This also is a trivial amount of work. Does kind of make me wonder why this project is almost complete, but not yet finished... Maybe they are selling under some kind of pressure.
I'm also curious about the bath/shower. Wonder if there is one. I can't say that I love some of their design choices, with the interior cabinetry/furnishings, but that may be just a matter of taste. I was also surprised that I don't see an axle replacement mentioned. The original axles have a lifespan of around 25 years, so if these are the originals, then they are way-ready (although they don't look to be sagging in the pictures).
Anyway, I was just looking at completed Ebay listings, and someone paid $8600 for a similarly old Argosy that was completely gutted, no work done, and just an aborted project. So $12k for a completely rebuilt trailer seems a bargain in comparison.
The challenge is knowning what "good" looks like. On the portal page, there is a "trailer inspector's checklist" as well as a utility to help you find volunteer inspectors in your area. There are lots of ways to replace a subfloor poorly. You don't want to end up re-doing a bunch of sloppily done work, or being stuck with what is known as a "polished turd," that is a trailer that has had some superficial fixes put in place but is fundamentally still a wreck.