A pullback: two different notions of the market

[timelets]

C - a coproduct of buyers, selles, bystanders, cops, dogs and all other creatures who showed up;
A - buyers who bought at least one product;
B - sellers who sold at least one product;

A -> C <- B

D = A x_c B - a cartesian product of successful buyers and sellers.

This diagram illustrates two different notions of the market: before and after. Probably, I need to rearrange it so that the time conventionally flows right.