If you buy something at $10 and sell it at $15, where are the $5 profit coming from? From the other market participants, e.g. someone selling to you for $10 and later buying it back for $15, losing $5 in the process. Your profit and their loss sum to zero, which is what "zero-sum" means.
It has absolutely nothing to do with opportunity cost, or whether you, personally, are a "loser". But if you're a "winner", someone else must be the "loser".
It has absolutely nothing to do with opportunity cost, or whether you, personally, are a "loser". But if you're a "winner", someone else must be the "loser".