First and foremost, NEVER let your PCs purchase stuff. At least nothing very magical. Purchasing herbs, maybe a few wands and potions, those are fine. But other things, wonderous items, staffs, magic weapons, magic armor. Definate no-no.
Secondly, while it's important to theme your games accordingly, don't be unwilling to try other sources IF you want to or you PCs do. Some work some don't.
Thirdly, try to encourge your PCs to think about where they want to go. Or even where you'd like to take them. Weight their abilities. Then see how it fits in.
Fourthly, don't be afraid to be a little mean and spontanous. Make them distrust arcane magic often, or at least not always reliable in every combat situation. Make them rely on the good graces of some clergy from time to time. Also keep things from being TOO black or white. Just because you work with a cleric of Chardun, doesn't mean you have to like him. By the same token, a druid of Lethene can be a useful party member, just a little...off.
Just a little side note though Blacky. I DO have an Ask the Sage post for Scarred Lands. Might want to consider that next time. K?