In a contract of sale, how are the buyer and seller determined? If we hold to the idea that all exchange is barter, then let us begin with a situation of barter to see the problem.
In the case of what I will call “pure” barter, that is where no property exchanged is used as money, some property is exchanged for another sort of property. I give you a chicken and you give me a pair of shoes. There does not seem to be anything intrinsic to this scenario that provides a ground for differentiating buyer and seller for I am buying shoes from you as much as selling you a chicken and vice versa.
It may be the case that I offer you a chicken for something in return or you request a chicken from me for something in return. In this way, we might say that I am the seller because I am soliciting a chicken for something in return or you are the buyer because you are seeking a chicken for something. These are accidental to the contract as such, because they are the extrinsic circumstances that give rise to the contract, but not intrinsic to the contract itself.
In a case of pure barter, some specific property is exchanged for another specific property, that is chicken for shoes. However, it is unclear how to view each, but the above does provide some insight. If I offer you a chicken for a pair of shoes, then it is unclear if I’m buyer or seller, because I am both offering and requesting specific property. However, if I offer you a specific property for some general return, then I am more clearly the seller. I am offering something specific for something general in return. In the case of a monetary exchange, the buyer has some specific property that he is exchanging for money. Though the seller desires money specifically, given the nature of money it is a general sort of property as property in general can be property. That this or that specific sort of property is used as money is irrelevant, because the buyer desires this or that specific property qua money and not this or that specific kind of property. In the case of a currency exchange, where each side gives money, the seller still desires property qua money. The buyer desires pounds or dollars specifically, but the currency seller desires currency generally, whether pounds or dollars. Moreover, the buyer is purchasing not only the currency but the labor, etc. specifically from the currency exchanger, whereas the currency exchanger is seeking money generally.