[jdmichal]

> Seems like all you can determine is that you once bought $x and once sold $y.

That's all that matters. Your gains (or losses) are $y - $x.

> it seems like there's nothing stopping someone from always taking the least expensive taxes first.

Yes. If you had some with gains and some with losses, you could even choose to sell those at a loss in order to offset other gains.