"What is my guitar (or mandolin, bass, banjo, amp, dobro, etc.) worth?"

Gibson Mandolins

In the teens, nearly every town had a ‘mandolin orchestra.’ It was a huge fad that lasted about 15 years. Gibson sold people the instruments AND the lessons. And they would often take a mandolin that someone was done with, return it to the factory (where it was re-finished), and then sell it again as new!
When the fad died off, they gave their best engineer, Lloyd Loar, the goal to revive people’s interest. Loar successfully redesigned the Gibson mandolin family to be far better instruments, but he could not revive the mandolin craze. Gibson mandos made during the Loar period (’22-24) are much better instruments and also fairly rare since people weren't buying, and all Loar's innovations had added to the price.

​Gibson let Loar go, and moved on to the next musical craze, mainly 4 string banjos.

How to judge the condition of a vintage guitar:

If it passes the dime test, (see video:

youtube.com/watch?v=rWEHPDI4F58&t=22s )

it likely doesn't need a neck reset and the "action" is good. That's a great thing, since neck resets are about $500 and require expertise. However after years and years of being under string tension, most guitars need resets.

Next, hold down heaviest string at 12th and first frets and look at gap between bottom of string and top of sixth fret with your string acting as a straight edge. There should be a tiny gap, like a business card height. This is the test is to measure neck straightness. Do the same thing with the highest string. The gap should match the gap of the lower string. If the gaps don't match the neck is likely twisted and that is hard to fix.

Many guitars (mainly Gibsons, since they invented and patented it) have a truss rod in the neck which, if functional, can help correct a bow, adding or taking away fingerboard relief.

Next, take page from a magazine and try to slide the corner under the edge of the bridge on the side opposite the strings. Often it is surprising how much of the paper will go under, and indicates the bridge is lifting and will likely need to be removed and re-glued. This is about a $250 job. If not repiared the bridge could eventually fly off and take some of the top wood with it.

Behind the bridge there is likely some "belly" in the top , where the top is swelled or risen. This could be a sign of loose bracing or just the natural body distortion caused by years of string tension. If it is not loose bracing it is not repairable and the guitar's value should be considered with this in mind.

Look in the sheen of sun for cracks. Cracks are always straight lines with the grain. Unless they were caused by an impact. They usually emerge from the edges of the pick guard or from the bridge towards the butt of the guitar. If it is straight line coming off the bridge, it is NOT a "finish" crack. Finish checking (loose spiderwebing or broken glass or grid looking) is normal and almost desirable as it indicates that the finish is original. Cracks are not not deal breaker but about $150 each to fix, and are always a "black eye" on condition. Cracks in neck or headstock are far more serious and can make the instrument un-sellable. Poorly repaired cracks are even worse as they are expensive to rework.

Is the bridge plate (under the guitar's top, directly below the bridge) original and in good shape? This requires an inspection mirror or a camera phone slid between the strings and pointing up at the underside of the bridge area. Original Martin bridgeplates are almost always maple and 1 to 1.5" wide. On older Martins the bridge plate is tucked into slots the X-braces at either end. Gibsons bridgeplates are often bolted in. (there are bolts under those pearl dots!) but never tucked. A replaced bridge plate on an old guitar can harshly discount an instrument as this area is the key place for sound/vibration transfer. Also, a replaced plate could indicate top damage beneath it. Large replacement bridge plates are tone-killers, as well as value killers.

Loar mandolins are a perfect example of instruments built "before their time." Loar was a classical musician and re-designed the Gibson mandolin for that style of music. When Bluegrass music was invented (by Bill Monroe and others in later decades), Loar-period mandolins were eventually recognized as the premier instruments for that style of music, and have been highly sought after ever since.