Price
The Price screen is used to price your inventory, this pricing does not affect how much money an NPC will give you for an item but when you stock a Player Vendor pet with these items they will be sold for that price when other players are shopping or using the marketeer system.

The price you put for an item applies to the whole stack, so if you have a stack of 10 arrows and you price them at 10gp that is how much the entire stack will sell for you can not sell the items individually unless you split the stack.

You have free range to enter any price you wish however when pricing items, try to figure out what you would be willing to pay for the item and if it is more than you have entered then ask yourself would anyone else still pay that ...