Brass sheets do sell, it just takes abit of time so you don't get your gil back for a couple of weeks. Or you could do the next skillup step and craft brass scales (13 goldsmith skill) which sell so slowly that it's like they never sell.
Second to Caladoern's statement. Farming the ores off of the worms in KRT while leveling (or farming) yields an insane amount of sheets, also getting your GS up past 10 almost free. The sheets do sell, very slowly, but can be sold for pure profit, making all transactions worthwhile if you're patient. Ingots certainly do sell faster, but cheaper. One of the best pieces of advice (From Shinanigans) was to go to KRT with a bunch of fire crystals, and copper ores (as they do not drop frequently enough to provide a 3:1 with the zinc.) and synth on site as your inventory gets full.
its so easy to make gil with goldsmithing in lower lvs it shouldent kill you to do some extra skill ups with ingots to sheets to scails.. if they sell slow find some one who needs to buy them