I found a better version of the movie’s story on Little Harlock Newslog:

Gekijoban XXXHOLiC – Manatsu no Yoru no Yume:
One day Yuuko receives an invitation by a mysterious addresser for the holding of an auction. A lot of hard edged collectors have been invited for this auction held at an old-fashioned European style building. However once Yuuko and the fellow invited collectors arrive the mysterious host of the auction doesn’t show up.
Moreover it seems that soon one after another the guests (the collectors) mysteriously disappear and even more strange events happen on the auction night.
What happens and who on earth actually is the host of this whole “party”?

Seems very, VERY interesting !

And about time, Young Magazine site has build up a section for the movie (click on the big banner), which is way better than the current official site. There are some bigger screencaps and character pics as well as some information, nothing much big, though.