It is important you know what you are selling, but equally important you let everyone else know. This is key. Wherever you intend to sell your guitar, be it Ebay, Craigslist, Gumtree or a private guitar classified website, there will invariably be a listing title. Get this wrong and you reduce the liklihood of viewers buying. Leave it out entirely (rather, leave out the keywords for which people search) and you dramatically reduce the number of viewers. Great listing titles will include the make, model, year and maybe other details too. For example
Good: "Vintage Gibson SG Special guitar, 1969 with case" - this will be easily found by all of those buyers looking for a vintage SG Special
OK: "Gibson SG, red" - although this will come up in many searches, it will be swamped by the many thousands of SG listings, all variants, new and old
Bad: "Gibson guitar, rare" - it is difficult to come up in search results with so few specifics.
The value of a vintage guitar is proportional to its originality. And it is well known that online sellers sometimes hide defects; posting poor quality pictures and claiming "I don't know anything about guitars" or "I'm selling this for a friend"... These tricks are well known, and in most cases will deter buyers. In some cases people will still buy, but assume there to be a problem, and bid accordingly. If you are not hiding anything, you should make this known! And truly the best way to do this is with large-sized, clear, photographs. Show the scratches, dents and wear patches. Show the electronics (and state potentiometer date codes if present) and the underside of pickups. You may also need to pay special attention to potential problem areas typical to that guitar. For example:
Gibson guitars can suffer from a cracked or broken headstock, especially those with a mahogany neck. There is a weak point at thinest part, behind the truss rod cavity, and many breaks occur at this point. Guitars like the Gibson SG often break where the neck meets the body, or at the tail end of the body around the controls and input jack. A really good repair can be difficult to spot in a bad photograph. You must show these areas when selling a Gibson.
The pickups in a vintage Gibson are crucial for it's sound. You must show that these are original. Usually done by removing them from their routes, and photographing the underside (with patent sticker), and the routes themselves (this also shows whether extra pickups have been added, and that the finish is original.
A Hofner may have a high action due to movement on a neck joint, or worse damage from a badly performed reset. Hofner was previously better known as a violin maker, and the hide glues used in violin construction were also used on their guitars. This allows easy neck resetting, but over time they also slip, making a reset a standard procedure for Hofner instruments. However if this has previously been performed badly, it can impact on the value significantly. You must show the heel area and the side of the neck (to show the action) when selling a Hofner.
Fender guitars were designed to be easily bolted together, and parts replaced when necessary. Very many Fenders out there have had components swapped over the years. Naturally buyers want originality, so you have to try and establish this where possible. A Fender neck should have date markings on the body end of the neck. Any serious buyer will want to see this. Also many Fender guitars out there are actually an old neck with a whole load of new parts added. The more places you can show - inside the neck pocket, under the scratchplate etc, the less room there is for confusion.
Guitars in rare or custom finishes are often claimed to be original, but very often have just been refinnished. To prove your rare coloured guitar has always been that colour, make a point of showing the serial number stamp - too faint suggests a refin; and underneath the scratchplate - finishes fade over time, but less so in areas not directly exposed to sunlight.
But pictures can't always tell the whole story. Sometimes you just need to state some obvious and some not so obvious facts, especially if you are unable to get decent close-ups, or if you are unwilling to dismantle the guitar.
Disclosing a guitars faults may not reduce the amount a potential buyer may bid, as it also tells them that you are honest, knowledgeable seller, hopefully reducing the chances of them getting a nasty surprise when the instrument arrives. This is also simple fairness.
For some guitars, most notably the guitars by Fender, but also Gibson and others, copies and forgeries abound. Fake parts or even whole guitars are passed of as original. Cheap Asian factories churn out authentic looking instruments that would only fool a novice. But then there are master fraudsters creating INCREDIBLY accurate guitars - often fitted with real parts. When some of these can sell for tens of thousands of dollars, it is no wonder. One thing these guitars never have is provenance. If you, or you grandfather, have played and loved this guitar for generations, show the pics! Cool old images of the instrument, howver grainy, prove it's history. Buyers find these fascinating and will almost certainly add value to the instrument. In fact you should copy these images, and include them with the guitar when you sell. People buying a vintage instrument are also buying a piece of musical history.
If you are selling the guitar with it's case you should certainly say so, and include pictures. Is the case original? And does it come with case candy? These are the things that came with the guitar: hang tags, price labels, manuals, catalogues, inspection cards, tools, straps and cables. Many guitars came with some or all of these, and having them present and in good condition are definitely a bonus to any buyer. If they are there, photograph them, and include them in your advertisement.
Be ready to answer questions. Hopefully you will have included everything in your listing, but if you have not, make sure you answer to the best of your ability. Failure to do so may look like you are concealing something.
Don't write "I'm selling this for a friend", or "I don't know anything about guitars". These are regarded as code for "there's something wrong with this sale, and i'm absolving myself of responsibility".
As stated earlier, prices change throughout the year, with January and June-August being the worst times to sell. The fact is, people have less money just after Christmas, and tend to be offline in the Summer months. A good time to sell seems to be November and December, when more people are online, and in a mood for shopping!
Just because your guitar seems to be worth $5000, don't get disheartened when it does not sell immediately for that ammount. There are only so many vintage guitar collectors, and most are a cautious bunch. Also most will not have big piles of cash burning holes in their pockets. To buy your guitar they may need to sell one of their own, or move around funds first. You will have to be patient, and expect to wait weeks, months or even years for the most valuable guitars to sell at the best price. A dealer will no doubt offer you a much lower price in this time, and you can sell to them, but naturally they need to make a living which will be reflected in their offer.
If you want the best return, you will have to be patient. Best to sell because you want to, never because you need to.
Sellers that put a little effort into their listings certainly reap the benefits in the long run!