Sale – this legal term is part of the Commercial Law in some jursidictions, where it details the transfer the right of ownership of an item, or valuable right, or of property, etc., from one party, called “seller” or “vendor” to another party, being “purchaser” or “buyer”. The act of “sale” is always done against payment of particular sum. This is why it consists of two opposite obligations – for the seller this is to pass the ownership right (including the right of possession) towards the buyer (with actual handling the item where technically possible), and for the buyer there is the obligation to pay the sale price to the seller.
Another ways in practice for exchange of ownership rights are for example the act of gift (donation), or when swapping items.
See also “right of ownership”.