Its one of several runes needed for various rune spells found here:
http://www.gamebanshee.com/dungeonlords/runemagic.php
Basically (once you have a rune spell) you place your cursor over the rune spell and it tells you how many and what type of runes you will need in order to cast that spell once.
Grymlock in the Northlands has most of the runes and spells that can be found in-game.
As to selling and then buying back at a lower cost.. thats because your honor attribute and/or your Bargain skill is high enough that you always make money (..and of course you make more money per transaction with more costly items like that rune).