Because book quests give out some gems as rewards, you can buy some gems for cheaper than it would cost to craft them. Things like Strength and Intelligence gems sell for a very low price most of the time. Wisdom and Agility gems are also from book quests but these don't sell for that low most of the time from what I've seen.
I'm not 100% sure since it has been a while, but I don't think you can't get Luck gems from book quests or at least from low level book quests I think. Because of that you need to craft them yourselves so buying these might be bad since the person that crafted the gem might be selling for a high price to make a profit.