Crafting mats

[Teiren]

While levelling, any gear you find with materia slots can be melded with up to 5 materia. But the chance to meld more than once past the given slots is so low, it's not worth it at all to try. The cost will add up very fast. Also, the answer Ichirai gave you about the potential of materia is something mostly unrelated and is outdated, no offense to him. At the bottom of the materia meld window, the game will now show the maximum amount of each stat an item can have. He is right that it's practically never worth the cost to meld a lot of materia, but you may end up with a few here or there, in which case you can easily use them with a Materia Melder NPC.

As for all the stuff you're finding, please take the time to look at its price on the market board and sell it there. When you're going to sell an item, there's a Compare Prices button at the top of the Adjust Price window, so this is a lot easier than you think. Plenty of rich people will pay a silly price for convenience, and when you're new to the game, any money is a healthy amount.