Well.. John made two trades.. one to Frank, and one to Sam.. so I think the article's right (" There's one buyer and one seller for every trade. ").

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).

Since I too enjoy nitpicking articles about nitpicking...

"Investors are fleeing the market."

Every stock is owned by someone all the time.

Sure, but the number of people that own stocks can decrease or increase. I don't have hard data, but I would bet that less people own stocks when they are low than when they are high, which would mean the phrase works as intended.

To me this meant "people who want to buy". It's easy for say 100 people to want to buy but no one to be willing to sell. It's of course a little silly as people would be likely to sell if buyers offered enough but I still think it's possible (at least in theory).

It is possible, yes, but it very rarely happens with publicly traded companies. It certainly happens much less than the phrase is used.

Typically people actually mean potential buyers and sellers. But, yes wrong when taken literally.