Sock yarn is a great thing to buy!
The amount you need depends on who you are knitting them for and what type of yarn you need.
Typically, to knit socks for a woman you'll need about 450 yards of fingering weight yarn, 380 yards of sport weight yarn, 350 yards of DK weight yarn, or 325 yards of worsted weight yarn. For men's you'll need 50-60 more yards in each weight. (ETA: I got this from the Senstational Knitted Socks book)
Fingering socks will take a long time! But some people like the feeling of the tiny stitches. Worsted and DK are great for thick, warm socks!
Good luck, have fun at the sale!