Factors that determine the value of a record include the mood of the market, the condition of the record and the pressing. Additionally, atypical factors that serve to make an individual copy unique, such as the autograph of one of the musicians involved in the recording, may increase the value.
Just as the latest music has an ever-changing market determining what's hot and what's not, the old record market also goes through phases that affect the value of entire genres. Last year, perhaps, classic blues was the rage, this year it might be western swing, and next year may see the resurgence of vaudeville.
Condition may be the most important factor in record value. No one is going to a pay much for a record, even one rare and in demand, if the record is scratched and the cover art defaced. On the other hand, even a very common record that looks and plays like it just came off the shelf still commands some value. Record condition is ranked on a scale ranging from Poor to Mint.
The pressing is equivalent to a book's edition or a car's model year. First pressings are generally more valuable than subsequent pressings. This is a function of scarcity, which plays a hand in every market. Pressing is determined by reading the serial number inscribed into the inner groove.
Finally, intangible factors make some individual albums more valuable than others. John Lennon signed a copy of Double Fantasy right before he was shot. Likewise, there exist acetate copies of God Save The Queen by The Sex Pistols that the laborers refused to label. Oddities such as these capture a mood or a moment and serve to make a record part of history. Naturally, this increases their value.