I am the leader of merchants,
Not sure on this one, but the first four letters are a start. Meh.
One of a kind
I am the only metal that is liquid at room temperature
Yet smallest of eight
smallest planet, since pluto got demoted
Look up and you'll find me,
...but not without a really good telescope!
Or down in the earth,
...as an ore, most commonly cinnabar, Mercury Sulfide
I'll tell you when it's hot or cold,
...if I'm in a glass thermometer, thanks to Herr Fahrenheit.
I was once used to heal,
Elemental mercury still has medicinal properties, however
Consume too much and you'll die!
...or slowly go mad, as a hatter. Hatters used mercury to soften felt, and absorbed it through their skin. Over time, the mercury would cause loss of mental function, similar to lead poisoning, leading eventually to death.
This is an edit.