Marking money from vintage instruments is possible. But it is job distinct from anything connected to music production and or performance. This doesn't mean they are not related. There is overlap in required 'sales' skills.
Attempting to select ten recent models that you sit on for twenty years to then pay for a child's college education is going to have the same approximate chance of success (and same relative costs) as buying weekly lottery tickets to accomplish the same goal.
You will increase the chance of success by increasing inventory above a certain hard to quantify but minimum level (certainly more then ten, but law of diminishing returns might kick in above 100, for a part time dealer), and increasing volume of trades with a similar ratio . . . to a certain level more trades enhance chance of success (if you learn something from each trade). Over the course of 20 years doing this you will, of course, appear to have sold the 'one' instrument that could have guaranteed early retirement . . . not following a similar strategy means you probably would not recognize the moment to 'sell' that instrument anyway.
You have to learn the minutia of the manufacturing and marketing process. You have to learn the minutia of who played what when and it helps if you can accurately (because these days much of it is easily verifiable on line) parrot it back instantaneously. While it helps to know difference between 'truth' and bullshit it is generally speaking variations on bullshit that sells.
It helps to be able to accurately track the market over a large number of years.
If one is a relatively honest dealer (oxymoronic?) a major, long term, expense will be to maintain a environmentally controlled storage: temperature, humidity, light . . . are all, at very least, significant. As is appropriate insurance.
A small private dealer needs to be conversant in human motivation and how humans respond to various cues. Simple 'good instincts' are seldom sufficient. A college foundation in psychology, and jaundiced view of 'sales' as a discipline is useful. As well as a clear understanding of your own motivation, and limitations. The 'value' of vintage is never intrinsic or obvious. It is not contained in the wood (I have a 'vintage' Teisco Del Ray', mid sixties, mahogany body, maple neck with rosewood fingerboard & an early (pre CBS) 60's Tele which presents a very similar 'value' in it's parts list. One is 'worth' substantially more then the other). Understanding the cultural forces that allow one guitar to be valued @ $10k and the other @ $400 is different then being able to manipulate individual motivation. I spent $25 for the Teisco & $300 for the Tele, neither were outright bargains, both were fair prices at the time. I'm unlikely to sell either, even if facing bankruptcy.) One of the biggest pitfalls to which a dealer can succumb is belief that they can influence directly the 'market'. Over the short haul, with appropriate insider info, get in, make a profit get out, never to return these ideas of dealer as magician can succeed for a few (and there are enough stories that it encourages everyone else). But stay at the party one day too long and suddenly you have a $10k investment (& a continuing investment, to protect original for facilities maintenance and insurance) that has an extrinsic but 'real' value of $8k. Suddenly you're stuck in the game for the long haul, behind the curve in terms of knowledge of other players, and needing to work at it like a 9-5 just to break even.
That said if your OCD impulses run the right direction, if you are a musician, collecting, auditioning, selling instruments can be an effective alternative revenue thread. Not too surprising the same type of environment for which you need to budget for performing, recording and mixing is quite similar to one effective for storing instruments & gear. If one is willing to accept modest profit to assist with cash flow (and if tailored correctly, even to assist with tax burdens), then making the turn over of music gear part of the long term business plan can be beneficial. And at some point in the long haul you are likely going to stumble on a gem that you will be willing to turn over for profit that will gain bragging rights at the local pub . . .
For me. I have yet to find any guitar, new or vintage, that off the rack has the sound for which I'm searching. They are all approximations that as a rule not only require a fiddling of knobs & dials but of geometry of intonation as well as specific parts. So? I sank a hundred dollars in new parts into the Teisco and now have an instrument worth between $70-125 . . . left untouched maybe double that (conservatively, based primarily on the fact that I am the anti-saleman). While I've had some people offer attractive chunks of change for the Tele (THE tele) that is always before a close inspection that would reveal what has been done to it in the intervening 45 yr.. (It is also exhibiting symptoms that will probably necessitate a full rewire). Fortunately, for me, it remains in toss up competition as the desert island electric with a musical value that far exceeds any monetary value. Modifying that piece of wood then altering it back is an inevitable part of using it as a tool to create individual art . . .
pretty much anything you do along those lines reduces the commercial value of that instrument, but it is the fact that individuals experimented with instruments in pursuit of unique sound that fuels the stupid collectable prices. While Santana might have played a Les Paul, in 60's, pretty close to stock and based partially on exposure to Bloomfield's playing, if he actually gigged with a Yamaha in the 80's it is unlikely that it had much resemblance to anything you could have picked up at the neighborhood music store. & the Les Paul is as good an example of this distance between musicality and marketing hype as any instrument or class of instruments out there.
It is a shame that entry level musicians are now, too frequently, pursuing guitars as fashion, but are then afraid to gut the instruments and change pots, change, resistors, experiment with different P/U's, different wirings, add a composite nut, a hunk of metal as a resonating block . . . majority of tweaks will have far less impact, superficially, then the latest Fx box (in increasing the range of sound) . . . but along the way you'll tend to discover one or two that subtly improve what you hear, that subtly improve what you project
of course there are two things wrong with my regret: majority of public does not want 'individual' sound & anything you do to tweak an instrument from it's off the rack appearance will ultimately (except if you become obscenely famous) reduce it's commercial value!
essentially instruments as 'tools' will not produce the direct financial reward as instruments as investments . . . the final catch 22 is that it is instruments as 'tools' that ultimately provide discretion concerning instruments as investment . . .
