Well, since most folks are chiming in with what
they're looking for, I figured I'd answer your
question:

Most folks display their cels for sale (with
prices or taking offers) on web pages hosted on
free hosting accounts (or similar), and then post
on forums like anime-beta.com or here or
rubberslug.com 

If you're selling a lot of higher-end cels, you
might consider running a silent auction on your
own web site (free hosts work well). 

If you happen to have any Gundam Wing cels in
there, feel free to drop me a line at the email
address posted above (just change the AT to @) ^_~