Who Made Who: Who made the guitar can make a difference. A generic guitar will cost less than one made by one of the big names.

Star Quality: Was it played by someone famous? Did a famous musician sign the guitar? Has the guitar appeared in photos or videos (creating a documented association between the famous person and the guitar)?

Details, Details: If the guitar is painstakingly handmade, has intricate detail work or inlays, or uses expensive materials it will cost more.

The Cabbage Patch/Beanie Baby/Wii Phenomena: If something is only made in limited numbers, it will create a buzz and a demand for the product. Because of the low supply, prices rise. A guitar with a limited production run will be worth more simply for the fact that only a certain amount of them were ever made.

Like a Fine Wine: Age is another factor. Older guitars used different materials and were more handmade than today's guitars. As practices became more eco-friendly the use of good wood such as ebony, rosewood, and mahogany in guitars became less common.

For Better or Worse: Just like other collectibles, a guitar in better original condition will most likely be worth more than the same guitar is worse condition. Of course, there is an exception to the rule and it is...

Keepin it Real: Modifications, replacements, and repairs will all decrease the value of a vintage guitar. A banged-up vintage guitar will be worth more than a refinished one that looks brand new.

So if you have a guitar that's in demand, hasn't been modified, and is in great condition, you've got yourself one sick (and most likely valuable) guitar!