When you buy any antique it will likely get passed down and or sold at some time in the future so having a proper bill of sale or receipt, is crucial to prove your ownership and ownership of a person's estate.
This form also serves to document and help to authenticate the item as a real antique by asking for more information than might be normally asked if a person bought a used item with no antique value.
How old is it? Year as close as possible with explanation.
Buyers will have more confidence when getting that piece of paper in hand as a receipt for the money they handed over and to show ownership of the item.
One form can serve for items that are part of a series or group collection although it might be suggested that for multiple items, multiple forms would be more appropriate.
If you are a buyer and you feel that the seller has only some generic receipt book type sales slip for you, then you may want to ask the seller to fill out your bill of sale so as to have better documentation.
As a seller, it is a good idea to instill confidence in your buyer so that you will have an increased chance at doing more business with that buyer.