Mold in 20 East 74th Street

All mold remediation is temporary until you remove the source of the mold's food (water from leaks, excess humidity, etc.). Mold is everywhere and cannot be fully removed, but you can certainly make the environment inhospitable for it's growth. If the reason for the mold can be determined and removed, then you should be good to go. Just reflect the costs of remediation in your bid.

(By the way, nice floorplan for 6D.)

Aaron2 is right - the fact that black mold is there means there was definitely a leak that is not addressed. It could be very costly to remediate. I would not buy until you have a licensed mold remediation company come over to assess and give you a bid.

A long but interesting read on toxic mold in residential homes:
http://www.nytimes.com/2001/08/12/magazine/haunted-by-mold.html

Think long and hard before you sign that contract.