Somewhat abstract, but, for me, I’d be willing to pay about $500 for the assurance that I don’t need to take the Fly to some Geppetto in order to get it in playing condition. At this point, I know what these instruments play like and, generally, what they will sound like in my hands, so I think I view listings based on whether they’re “a project fixer-upper as-is” Fly or a “this one is ready to go” Fly. @Dingus’ recent listing in the Classifieds, for example, is very inviting because it seems like I’d know what i’d be getting into.
I arrived at $500 (above) by estimating the things I know that could be wrong with a Fly without the seller knowing it and 1) how much I’d need to pay to fix them or 2) how much I’d lose in putting the instrument back on the used marketplace while disclosing the shortcomings. If a listing said “here are the numbers on the action with a set of D’Addario 10s, and I promise you everything works”, I’d be inclined to believe that seller and then I wouldn’t need to wonder what sorts of headaches I’d encounter down the road if needed to make an adjustment in order to accommodate a different gauge of strings.
I don’t know why anyone would be inclined to buy a Fly because the listing says “rare” or “prices are starting to rise”, but i don’t view guitars as investments or decor or collectibles; no judgment intended, however. If I saw a koa Fly for sale, I’d want it for the tonal color and not at all for the finish color.
I do think I’m programmed (to some degree) to think that a deal for a beat-up Fly is around $1200 and any Fly above $3,500 better be really special!