Quests require you to have a certain level of fame. The higher the fame, the more quests you can do and thus the more rewards you can get. If I'm not mistaken, the higher your fame the cheaper items are in the area if you buy them from an NPC.

For San d'Oria, buy loads of millioncorn. You can either get it at the culinary guild in Windurst Waters or from a guy in whatever country has control of the Argeunau (sp?) region.

Then take it down to Selbina and trade it in to the guy by the sheep pen. You'll lose a bit of gil, but relatively little. I'd buy something like 20 stacks and bring'em down to Selbina. I'd only lose like 4k or something like that. It also gives you Bastok fame if I'm not mistaken.