Before you ask, there is no such thing as an absolute guitar value for vintage instruments. Sorry. Online guitar prices go up and down over the years, and even within a year, with prices falling off in January, and again in Summer. But you can make a few generalisations:
Following this logic a 1960 Epiphone Casino (early period, US-made by the craftsmen at Gibsons Kalamazoo factory, associated with the Beatles and the Stones) is worth massively more than a 1970 Epiphone Casino (70s, Japanese-made, average quality, aimed at students). Probably ten times as much.
Whatsmore, values vary from country to country, from city to city and from year to year. And then there is condition. A vintage guitar that was bought and stored unused ever since is a very rare thing. Mint guitars like this can sell for very high prices. A beaten up guitar with replaced parts, a new finish and structural repairs might be worth very little, especially if it is not the kind of thing people chose to play.
So although there is no such thing as an absolute value, guitars do tend to sell for something approaching a consistent price. A number of people follow the market and put their research into various guitar value books. These do give ballpark figures, though far from absolute, but they are a guide to whether a guitar is desireable or not - see vintage guitar price guides for some more information about these books.
And you can do this research yourself. One easy way is by searching completed listings on Ebay; prices of sold items are listed in green, unsold in black or red; each is relevant as you can get an idea of what sells, how quickly, and what doesn't sell at all.
Vintage guitar stores sell guitars typically at 30%-50% more than a private sale from a classifieds advertisement or a website like Ebay. Perhaps more still in a large city with a super-rich clientele. Many online stores offer guitars for sale at a considerably higher price than they expect to sell for; they might just get lucky, plus they have room to negotiate with a potential buyer. But many have guitars sitting in their sales rooms for years. But this method can also work for the private seller who does not know the value of their guitar; as long as they are also in no desperate hurry to sell. Ebay allows sellers to offer an item for sale at a fixed price, and accept offers. This will ultimately give an idea of value, probably a shade under what it is actually attainable.
When you sell online you have a vast reach - someone, somewhere wants your guitar. It is just a matter of making sure they see it and are willing to pay for it. You can't control how many people want your guitar, or how much it is worth, but you can do a lot to make sure they see it, and it is presented in it's best light; see part three Advertising your guitar for sale.