Well the first tip to sell something is:
Never sell a useless thing for a ton of money.
Tip number two!!!!!!!!
Only sell something for a ton of money if it is like a Enchanted Diamond Sword with good Enchantments Or a Enchanted Bow with good Enchantments
Tip number three!!!!!
Only sell Packs of 64 for alot of money if it is 1 thing make it only for 100 but if it is some MAJESTIC DIAMOND make it for about 1.000
Well here is a picture of the market thing to see it type /Market Listings in ya chat and here is what it might look like but it might look different because someone might have brought the stuf: