If you do not intend to involve a professional vintage guitar salesman then there are a few things to be considering in your travels: the year of production, the exact model of the guitar, the condition that it is in, the originality / rarity of the batch that it was released in (this includes any unique defects a guitar might have) - sometimes even the color can have a considerable effect on the overall price of a guitar, whether to its advantage or to its detriment. Many of these things should be readily apparent to you, though do not be afraid to use the internet to find out any of the particulars, as this is what collectors feast on.