i would definately price everything. Think about how you would feel if you were buying - the seller could just make up a price on the spot - personally, that always puts me off. A simple sticky label will show that you have thought about what you want. Also, if you are selling good clean stuff, you may get pounced on as you are setting up. Labelling the night before means you won't have to think of a price whilst flustered.
As the day goes on and you are left with things, you can pop a sticker over them or make a "everything half price" poster.
It really depends on whether it's a one off. If you are doing the sale because you want to get rid, then price very reasonably. You will find that people will buy more from you - and probably come back after seeing the other stalls that are over charging.
I recently paid �3 for a girl's coat, �1.50 for a boy's m&s jumper and �2 for a leather handbag.
My son sold his light sabers recently - he wanted �3 each. Many people came and looked, but walked away.
He made a little sign "�3 each or two for a fiver" and he sold all of them within half an hour.
I would also say think about how much you paid originally and the condition. You may find that the other stall holders will buy a lot of your stuff, to sell on at a dearer price. But, in my experience, they are the ones who will still be with it at the end of the day, carting it back home again!
I hope i've been of some help and good luck!