According to Bob Reed, the chief numismatist at RR Rare Coins & Currency, a $2 bill of any kind is legal tender and is worth $2 if spent. However, red series two-dollar bills are sometimes bought individually for $9 to $15, depending on their condition, and are found for sale online or through a collector.
Mr. Reed explains that two-dollar bills with red ink, called the red series, were printed in 1928, 1953 and 1963. The earliest of these were part of the United States' first run of smaller paper currency, a move introduced in order to save money on printing costs.
Mr. Reed also mentions that a 1928 red seal pack of two-dollar bills from 1928 is worth up to $20,000 in 2014. Bought individually, older bills with a seal in red ink are collectible items and are typically worth more than $2, depending on their condition. The value of a bill rises considerably if it is in crisp condition and part of an uncirculated pack of bills. Collectors use the coin grading system to decide the value of the bill.
The Numismatic Guaranty Corporation notes that the grading conditions are named Poor, Fair, About Good, Good, Very Good, Fine, Very Fine, Extremely Fine, About Uncirculated or Mint State. Bills and coins are graded on a scale of one to 70, with 70 being the highest score. Mint state bills rate a score of at least 60.