You want to make 10,000 gil easily? Well then go to the Treno auction and buy the Griffin's heart on disk 4. After you do that, go to the adventerer wandering by the Members Only tea party. Talk to him and he first will offer you 8,000 gil, select to No Way! he'll make an offer for 10,000 gil. Thats the highest amount you can get for griffin's heart.