You can ask for it seller, you can ask them for the lowest price or you check the auction house to find out a price of a item.

Try to sell high, you can still make the price cheaper after you tryed to sell something high.

If you wanna sell a items you can ask the purchaser whats his highest offer, than you can deal a price in the middle.
is the offer of the purchaser to low, keep the item and try to sell it a view days later again.