One thing to remember is that you don't know
exactly how much they overbid you.  With eBay's
system, if they bid $1 more than you or $1000
more than you, they still get it for $1 more (or
whatever the min bid increment is).

On the subject of "why even bid?", I see nothing 
wrong with bidding the max that you will pay, 
even if you don't think you'll get it.  First,
you never know what the other guy has bid.  I've
gotten cels for half of my max when I was sure
I'd be outbid.  Also, you have to consider the
opposition at times.  Suppose you and another
collector both constantly bid against each other
because you both collect the same char/series and
the other collector has a bigger budget in
general.  Should you then rollover and let that
person get nice cels for next to nothing anytime
you see their name on an auction?  If you do so,
then they'll still have lots of money to spend
the next time that char/series comes up, whereas
if you bid the max you will pay, it'll eat into
their budget for the next auction where yours
will be full.  I'm not saying to bid more than
you normally would, but there is absolutely
nothing wrong with bidding the full amount that
you would pay for a cel even if you're sure you
would lose.