Hi,
I have had that problem also, I filed a support request, the reply was not very useful, but they did tell me they had purchased the editor from http://www.fckeditor.net/ and that there may be more info available from there.
Basically I have to insert the center tag like you are doing, but it needs to be as close to the item you want centered as possible. Experiment with placing it at different locations.
Here is an example from one of my listings, for a picture...
</font></p><p align="center"><img alt="" src="http://img.auctiva.com/imgdata/1/6/6/3/4/9/webimg/18677717_o.jpg" /></p>
Ricky