Regarding your hypothetical(?) "Vintage Yamaha Acoustic Guitar", as was mentioned, you need to know the exact model you have. It should be marked somewhere on it, probably a paper label glued to the inside and visible through the sound hole. Rarely will you find the model stamped into the wood on the back of the head, or as part of the logo decal on the front of the head.
However, this interior paper label may have been removed or fell off, depending on how "vintage" it is and what it's been through. These labels are applied with a respectable glue so as not to fall off under normal conditions an acoustic guitar would be exposed to. If it's been exposed to high humidity and moisture in general, and especially enough to degrade the paper and/or the glue holding it, then what you have is firewood. Selling it on without that information would yield a well-earned negative.
As for determining which price suits yours, always check the Sold Listings for your exact model number. Never judge the value by Active listings. The only true barometer of what someone WILL pay (i.e. "worth") is what someone HAS paid (i.e. "market value"). Note that this range may also be drastic. Some people have more money than brains. Shills still exist. And sellers who don't know what they have and cannot spell properly will have their listings buried, and a savvy searcher will encounter a diamond in the rough and snatch it up before the other treasure hunters find it. I've seen USA-made guitars go for less than $400 with a single bid due to these basic seller mistakes.
So, the model you have should show up in the Completed Listings. That alone doesn't mean it SOLD, so you have to also check the SOLD ITEMS box.
If you don't find your exact item there, that simply means one hasn't been sold within the history of that search algorithm. Don't assume the next model number up or down is the same thing and base your pricing on that, as model number designations can denote a very dramatic difference between those models, and you'd either sell it for much less than it's worth, or buyers will peg you as an idiot for trying to get "next model up" prices.
Your best bet there is to post pictures and ask questions on community message boards for that brand and general instrument type (Yamaha put their name on every known musical instrument; accordions to zithers.)
If they'll give you an honest answer as to model and possibly year, and, if you believe in miracles, a current market value, then you'll be set. Otherwise, you put it up with as much info as you have, and let the bidders have at it.
