You don't need to subtract a penny to make it so. The value of the product for A must be greater than $13 or A won't buy it. The value of $13 cash must be greater than the value of not selling the product to the consumer or the seller won't sell. Everybody has more value than what they started with!
The only way this can possibly be true for all cases is via per-consumer price discrimination, anything less will always leave someone out.
The value of the product for A must be
greater than $13 or A won't buy it.
If there's a price where I will buy (value to me > price), and a price a tiny bit higher where I won't buy (value to me < price), there must be a point in between, where I am indifferent to buying (value to me == price)
For example, if I'm a trader and I know I can sell a widget on for $13 (after all costs are taken into account) and a supplier offers it to me for $13 I don't lose any money by taking the deal, but I don't make any money either. So I don't care if the transaction happens or not.
I'm of the opinion indifference is a temporary state of mind that, when encountered, forces one to decide one way or another. If it wasn't, the binary question "would you like to buy this" is unanswerable, as if in some perpetual superposition of yes/no.
This, evidently, doesn't ever happen in the real world: in reality, you'd take into account whether you have time to buy it, whether you have the will to carry it around, etc., before arriving at a definite answer that implies (value - price) has collapsed to some non-zero value.
You don't need to subtract a penny to make it so. The value of the product for A must be greater than $13 or A won't buy it. The value of $13 cash must be greater than the value of not selling the product to the consumer or the seller won't sell. Everybody has more value than what they started with!
The only way this can possibly be true for all cases is via per-consumer price discrimination, anything less will always leave someone out.