New items, Quantity, Day of the Week. These are the three main factor to think about when selling on Marketboard.

Patch Day is the big money makers because new items mean rare with no stock. Everyone wants it, nobody has it.
Quantity is your basic supply vs demand. More stock, lower price.
Day of the week falls under quantity to a degree. During weekdays there is less stock so items usually can sell higher. Weekends is when everyone plays so you see prices fall during this time but items sell faster.

Personally I adjust prices on Friday and Sunday. I don't need money fast so I can leave items on the board and wait for stock to get lower. Sooner or later it sells.