When I set the following find member filter field:
_MM_POSTSMORE => 'Number of Posts is greater than'
to 0, the member list shows members which have 0 posts and not only members which have 1 and more posts.

As far as I can rate this, it's because of the line 301 in modules/xoopsmembers/index.php:
if (!empty($HTTP_POST_VARS['user_posts_more']) ....
maybe you can change this to: if (isset(...

This is true also for some other vars in modules/xoopsmembers/index.php and also I find the same code in modules/system/admin/findusers/main.php

Thanks! I have just fixed this on CVS.