I absolutely do not recommend using a marker.  If
I were you I would make a correction layer instead
of putting anything on your actual cel, this way
it's much easier to fix any mistakes and it's not
permanent.  

I use Rapidograph pens with Bombay India Ink on my
fancels and correction layers.  If you are making
a correction layer you should buy Dura-Lar
acetate.  I use .005" thickness for my fancels,
but for a correction layer it is probably best to
go with an even thinner acetate.  I've put the
links to each thing down below but you can also
get the Rapidograph pens on Ebay for a much lower
price (especially if you are buying the set).  

Rapidograph Pens :
http://www.dickblick.com/zz210/03/products.asp?param=0&ig_id=1395

Bombay India Ink :
http://www.dickblick.com/zz211/22/products.asp?param=0&ig_id=4218

Dura-Lar :
http://www.dickblick.com/zz555/06/products.asp?param=0&ig_id=1597

Also, you might want to check fancel tutorials as
well, they have other recommended pens and ink.  A
good one is here :

http://bakaneko.donthurt.com/index.php

The things that I have listed I've used and they
work great.  It will take some getting used to and
many mistakes (like in my case ^^) before you are
comfortable and can draw straight/smooth lines.

I hope this helps some ^^;