I'm not familiar with Brooklyn property, so I gladly defer to the other answers provided by those who are.
However, just a few more general thoughts:
Sometimes comps ARE difficult to come up with. That's where a Realtor familiar with the area can be especially helpful. Sometimes, for instance, you may have to extend your search geographically. Or look back more than 3-6 months. And it is a hassle when there are foreclosures mixed in with non-foreclosure sales. In those cases, you do the best you can. One thing your agent might do (depending on the protocol where you are) is to ask the listing agent: "How did you arrive at the price you're asking?" Although the listing agent has a responsibility to get the most possible for his/her client--the seller--the agent doesn't want to overprice the property, either. So many listing agents--especially in situations where comps are difficult to come up with--don't have a problem showing (and justifying to) the buyer's agent the justification for the listing price.
Another comment: When you're dealing with a mixed-use property (residence plus rental, for instance), sometimes you have to break the two apart. Look at the value of the residential portion. Then look at the value of the rental property.
And just an additional side note: Mitchell offers some very good advice, but I respectfully disagree with him on one point. And that's how to use a multiplier to determine value. A lot of experts do just as he suggests--both in valuation of properties and in valuation of other things, such as business valuation. But what really matters isn't the raw (or gross) rent rolls. What matters is the return on investment.
Simple example: You have 2 buildings, each with rental income of $50,000 a year. Using a gross rent multiplier (let's pick "10"), that might suggest each property is worth roughly $500,000. However, let's say one is individually metered, with the tenants paying the utilities. The other isn't; that's the landlord's responsibility. Let's say one is in good condition, and is running about $5,000 a year in maintenance and repairs. The other one is running about $10,000 a year in maintenance and repairs. One has some pay washing machines and dryers in the basement--yielding $500 a year in net profit. The other doesn't.
As you can see, one building is likely to be more profitable, from a rental standpoint, than the other. So the real question isn't how much in gross rentals the properties bring in, but what the net is. Then you apply a multiplier to those numbers for a more accurate valuation.
Hope that helps.