just use your mind a little

learn a crafting class

craft easy things that can be used to craft equipments ( not equipments itself ) like leathers from leatherworker and sell them

i have managed to gain over 70k gils while im still lv27 leatherworker within 3 days

lack of mats needed to craft them buy them from market you will get all mats with half the price of actual thing you will sell

also when making a price try to be cheapest or 2nd cheapest

also sell them by 10 for each transaction unless its pretty cheap and doesnt exceed 5000 gil in each transaction unless its a very rare mat

most ppl try to save gils so dont expect anyone to buy 99 from a mat that its average cost 200+ gils

hope this helps