With 2star/3star materials obtainable easily in dungeons, trading GC seals for those materials aren't so worth it. The only thing that probably worth it would be Cordials (500 seals each) which can be traded.

I would suggest crafting and gathering for the Ixali tribe dailies so you can get Ehcatl Sealant to sell everyday but since you don't have them up according to your lodestone profile. like Yhuvix said, spiritbonding or farming T4 for allagan materials will be your best option to get more gils. You can purchase ilvl45+ gears from NPC in mordhona / north thanalan to spiritbond into grade 3 (and possibly 4) materias.