Feedback for gauging rental demand and looking at rental comps

[Priscilla Chin]

Hi everyone, 

I'm looking to buy my first long term investment property and am not sure whether I'm approaching my research for rental comps correctly. Please give me feedback!

First, I type in the zipcode to look up the rental statistics on zillow. Example with zipcode 11203:  https://www.zillow.com/rental-manager/market-trends/11203/

Then if the market temperature on this page says warm or hot, I'll look up a house I might want to buy in that zipcode. Example: 3bed 2 baths. And I look up 3 bed 2 bath houses for rent in that same zipcode to see houses similar to the one I chose to get a gauge for how much I can rent it out for. 

Along that process, I will come across a few homes in that zipcode that have been on the market for a while...64 days, 96 days, etc. These houses seem to be in decent condition, clear photos, etc renting at a reasonable price compared to the newer listings. When I see this, it's concerning because.. why are so many decent looking homes in a "warm" market temperature on the market for two plus months??? And that usually steers me away from that zipcode because I have to imagine that my house can possibly be vacant for months as well. 

Is that conclusion correct? Am I missing something? Are there other ways to gauge the rental demand??