First to start off you need to check market boards to see what is hot and selling.
Never look to guides what to sell, the board changes every week on what's needed.
For example, silver in siren can be in demand for three days for 800 Gil each. The following days it can go down to as little as 200 each because people try to down the last person.
Here is an example of what people do to get someone to buy their items, NEVER GO MORE THAN 100 GIL BELOW THE LAST PERSON!! Every bit counts.
Also dont forget to see how frequent the items sell for, items that sell daily and cheaper can make more than a higher priced yet solemnly bought item.
Materia usually sells on market boards easily, just click on materia tab and see what materia people are looking for. This is usually gathering, slaying, heavens eye etc. Figure out what sells for the most.
To obtain materias you can get your retainers on a venture (you can get ventures at your grand company salesman, leve quests or beast man tribe quests). Along with the beast man tribes, you can buy materia from them for their currency you obtain through their quests. Or if your far enough into the story, rhalgres reach has a sales gobby that takes cracked clusters from your daily roulettes.
Other materials on the board could be cloth and armory, armor you can obtain in certain dungeons (not all dungeons have traceable armory and gear). Or you can obtain armor from main story quests/crafting.
Cloth can be crafted cheaply, or you can buy them from beast man tribes. Depending on what tribe it is, Sylphs sell very good materials like Linen and cotton. You don't need to do much besides the first few quests to unlock the trading. Almalja sell ingot materials, find which one sells the most in your markets and see if it's worth to buy a full stack.
Ex: Sylphs materials
Don't forget, you could always change up the market board by buying out cheaper items and selling them for more.
SPEND MONEY TO MAKE IT