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.
I am
Mercury.
.
This is an edit.