When I visited Pisa, my Art History teacher pointed out a large marble "brick" on the exterior that had inscriptions carved into it. She claimed that this was a pagan sarcophagus: They simply dumped the body and re-purposed the marble. Is this true, and is there any more information about the practice, or the individual marble pieces? This is the only thing that I can find, but I didn't see her claim substantiated.
Your art history teacher is right! Lots of late-Roman and Medieval buildings were constructed using stone moved from older structures - a process that historians call 'spolia.' It was a common phenomenon, and makes practical sense when you think about it. In that period, people didn't necessarily have the funds or the technical knowledge to quarry new stone, but they did have access to lots of old buildings, no longer in use and made from high quality marble. It also allowed them to appropriate the past, taking some of the glory and grandeur of ancient Rome and attaching it to their new structure.
It's not even just an ancient thing. In my home town of Reading, one of the main churches (St. Mary's) was repaired in the 16th century with stones robbed from the abbey - recently dissolved by Henry VIII.