I believe a lot of no-fee apartment complexes in NYC offer a referral fee to the website that refers a lease signer, which is at least part of the reason that a lot of apartment sites deal exclusively in large apartment complexes. This isn't charged directly, but amortized in generally higher-than-average rents.

But did you expect that this startup wouldn't be planning on any money by playing the middle man?

Eh, maybe not now, but they were apparently picked up by YC - they probably have at least some novel ideas. I just had a look at their NY Times article, and they seem to have at least one - setting up appointments with the realtors online.

But yeah, I know the NYC housing game is pretty brutal, I've played it too, unfortunately.