Outdoor items at estate sales typically need special consideration when marking their prices. This is due to the elements and other factors associated with outdoor items such as material quality and degredation such as rust.
Tape and stickers are not recommended due to dampness or rain and dirty surfaces may cause the adhesive to peel off after a few minutes. So instead of using sticky labels try using tags with an attached string and use a bic ball point pen or other indellible ink to write the price on the tag. Markers and some types of roller ball ink can run or vanish completely if they come into contact with water.
Using tags on estate sale items is also beneficial because there is more room to write the item name or a tracking number if you use an inventory system. We know it's not always possible or reasonable to use tags on all items but definitely use them on the more valuable items at your sale so that you know the item is correctly priced when it is brought up for purchase.