Actually, funny you should mention this, since
that's what I ended up doing last night (though
not for a cel.  It was for a fairly low-price
item, doujinshi).  I'm with Cres on this one; you
bid X amount, then look at the auction, see that
you're still outbid...and oh, maybe just /one/
more dollar.  Hmm.  Still outbid.  Well, how upset
would I be if I lost just by one dollar?  I mean,
it's worth one dollar more to me, right?  Let's
put that in and see.  Hmm, still outbid...

It's not really about beating out the other person
by a dollar or driving up the price, it's more
about 'well, I don't want to /lose/ it by just a
dollar' ^_~.  At least for me.  I don't bid very
often on eBay any more because I generally have
better luck finding the rare items I want on
obscure websites, and because watching the ends of
auctions where I'm the high bidder is liable to
send me into hyperventilation fits.  I get really
jittery.

Another way to look at it (as well as more
mercenary) is that you don't know if the current
high bidder is watching the end of the auction. 
If you increase your bid by small amounts, then
you keep the high price down as low as possible. 
Hence, if the person who was the high bidder comes
back and outbids you, the final bid is still as
low as it could be, and maybe they won't have time
to outbid you again before the auction closes.

...heh.  So speaks the person who feels incredibly
guilty whenever she outbids someone.

I've been on the other side of the fence too,
where I've been the high bidder since day one and
someone tries to slowly outbid me at the very end
of the auction.  I generally just smile to myself
if there's no way they're ever going to reach my
high bid, and then spend a moment at the end to
daydream that I could have gotten it for X amount
if only that other person hadn't come along.  

But such is the nature of online auctions.

Talia, who is aware that very little of this made
much sense.  Chalk it up to the fact that I've
been participating in online auctions for far too
long, and have some really weird instincts in
place ^_~.