Last year we used and were very impressed with Vacanza Bella, http://www.vbella.com I don't think they have any city apartments in the Marche, but they do have a villa or 2. They have many city apartments in Tuscany. It is a first-class operation and I would use their services again without hesitation.

Another broker we've used in the past is http://www.rentvillas.com which was ok.

A great source of info is http://www.slowtrav.com , which another Ziner pointed me to last year.

Carol in Boulder Colorado