Answer

For recent sales of condo units in specific properties, active NYU students can ask Jack Brause Library staff (midtown.library@nyu.edu) about mediated access to the PropertyShark database. The NYC Department of Finance ACRIS database also allows anyone to search for financial documents associated with real property transactions, including condo sales. Some sales trend information can be found in market reports published by a variety of brokerage companies.

Researchers interested in historical condo sales data can also consider the following (no-longer-published) publications:

Yales Robbins Condo Sales Report (1989-2009)
The Redbook (1992-2012)
Real Estate Directory of Manhattan (CoreLogic, formerly First American Real Estate Solutions)

These resources are available at the Jack Brause Library, NYU Midtown Center