New York City Housing Data

It's amazing what you can find online nowadays.
When I was looking at that studio apartment, I was told at first that the seller had lived there for about 4 years. But I was able to use Property Shark to discover that he had actually only lived there for 2 years. To do a search, if you have the address of the property, start at Property Shark to look up the block and lot number. You can also see all kinds of information right there-- size of the lot, a map of the location, building violations, certificate of occupancy, tax assessments, title history, etc., sometimes even mortgage amounts. Unfortunately, for co-op apartments you can only see when the apartment changed hands--transactions are recorded differently since it's technically a sale of shares in a corporation rather than a sale of physical property.
Once you have the block and lot number, you can go to ACRIS and see actual scanned documents. Again, for coops, not much detail-- but for condos and houses, you can usually see how much of a mortgage was taken out to buy the property. Even if you can't see the actual selling price, that is a great thing to know if you are planning to make an offer on a place.
I was able to discover that my landlord bought the building I live in in 1998. It's supposed to be a 3-family, though he has actually divided it up into more apartments than that. There were citations for no hot water and exposed wiring at various points over the years. He took out a mortgage of about $130,000 to buy the building. This made my jaw drop at first, but then I remembered that it must have been pretty much an SRO at the time, and in really bad condition. One of the original tenants was still renting a room (that didn't have a kitchen or its own bathroom) when I first moved in. But now my landlord has renovated the whole place from top to bottom. This building is assessed at around $600,000 but I've heard of similar places going for around $1 million lately.
I'm not sure what I'll do with this information, but it is nice to know it's out there! (And if anyone knows how to find out the mortgage amount or sale price on a co-op, let me know!)