rear main seal leaking questions ?

[smitty619sd]

Ok...... Im looking at buying a 98 4runner 5 speed with 100k that has a rear main seal leaking.

My questions are ...

* Why is this leaking at 100k, is that normal ?
* Dose this tell me something about the way the truck was drove ?
* If i don't get it fix asap will it cause more problems ???

Thxs