Seals for instance can allow you to buy the items to craft the "token items" IE Potash is used to make Vanya Silk. Vanya Silk is used to trade for 3 Star books. So, Weavers would want to buy potash to make vanya silk to turn it in for the book.
Keep in mind though, to buy these materials, You might have to rank up to the last rank of your Grand Company
Im not sure for the soldiery items but, what you can do is search each item up on the marketboard, see how much are going for and at what price each were sold for, and sell the one you think will sell the most.
and as other have said, you can probably net in 5k-20k gil daily depending on what you do. Don't forget that the Challenge Log can provide a good amount of gil weekly. Remember, spend on what you need and not what you want LOl. If all you care for gear and progress, Gil isnt really "needed" unless your attempting novus
