I guess it depends. Some buyers like the instant gratification of "Buy it Now." On the other hand, if you have an item that is rare and several people want, you might be able to get a lot more if the bidders fight it out in an auction format.
I suggest you try selling some simple things first to get a feel for how it all works (CDs, socks, etc... lol). Then try your plates once you are comfy with the process (and have a higher-than-zero rating).