> Because for every person trying to buy it for X someone is “buying it” for -X.
Selling “it” for X is not buying “it” for -X. It's buying X for “it”.
More specifically, if A sells X to B for Y, A gives up X for Y implying Y is worth at least X to A (and X at most Y), While B gives up Y for X, implying that X is worth at least Y to B (and Y at most X.)
Hence, that trade occuring in market where X and Y have access to all potential purchasers and sellers of X and Y fixes the value of X equal to Y.
Selling “it” for X is not buying “it” for -X. It's buying X for “it”.
More specifically, if A sells X to B for Y, A gives up X for Y implying Y is worth at least X to A (and X at most Y), While B gives up Y for X, implying that X is worth at least Y to B (and Y at most X.)
Hence, that trade occuring in market where X and Y have access to all potential purchasers and sellers of X and Y fixes the value of X equal to Y.