Translation:Shulchan Aruch/Choshen Mishpat/201
Paragraph 1- If one sold with just words and they set a price and the buyer made an impression on the item so that he would have a known sign that it belongs to him, even if the buyer did not give any money any party that would retract would accept “mi shepara.” If the local custom is that an impression is a complete kinyan the item would be acquired, neither party can retract and the buyer would be required to pay. This assumes the impression was made in the presence of the seller or the seller told him to make an impression on the item.
Paragraph 2- The same would apply to any item that merchants have the custom to acquire with, such as the buyer giving a perutah to seller or through shaking hands or in a place where merchants have the custom to give the purchaser a key. The same applies to anything similar.