If as a seller you see an obvious typographical mistake in a bid..

i.e.: listing a bid for $125.00 instead of $1.25

A typo can happen to anyone, but it could easily slip by and shake up a new member who could be at a loss as to how to deal with the problem.

It only takes a few seconds to zip off an e-mail to ask "Did you really mean this bid?  The good will you gain as a result will be worth much more than just sitting back and gaining  a few dollars as a result of their mistake.