The expression is normally used to comment on the overall market behavior for the day (or some other time frame), not a single transaction, so the whole "There'

The expression is normally used to comment on the overall market behavior for the day (or some other time frame), not a single transaction, so the whole "There's one buyer and one seller for every trade" thing is somewhat irrelevant (edit: what I'm trying to say here is that John shouldn't count as two people just because he is involved in two transactions since the statement applies to the aggregation of all transactions over a day or other time frame). Also, from John's perspective it may look like a single trade (he won't know that a computer matched his sell with multiple buyers unless they buy at different prices).