That's a hard question.
Ways to find value range from free to paid and the difference is normally the time it takes you to get the real data.
I use PME Blackbook (also known as REI Blackbook), but that's a paid service, and that works in NY. Another good service is realquest express (freeish).
Otherwise you can look at solds in zillow and trulia, but I don't like to use them much. Although they have gotten a lot better in recent years. You should look at zillow because the people selling their home will, and you need to be able to defend your opinion from that if zillow is not accurate.
Also, I think NY has that as public data that you can view online.
The best thing to do is call up some appraisers in the area and offer to take them out to lunch or dinner and ask them how they find the deals (or find an investor friend and on one of their appraisals, the appraiser probably put his source for his comps) and hopefully they are not just using the MLS.
Also, it is always worth it to put a property under contract as long as you are clear with the seller what you are doing. Your needs and their needs have to match and if they do, then you can work it out so that they work with you because of all the value you bring. If they are not motivated then there is nothing you can do in the first place as an investor. Don't let unmotivated sellers eat all your time, that's what realtors are for (and another reason to get a realtors license).
The way that you figure out if investors will be willing to pay for it is to find out who are the guys that are buying and just ask them. Look for a list of properties that are purchased that does not have the house itself as the mailing address. You can find a bunch of list brokers online (reisource.com , so on) or you can often have a title company pull you a list (as long as you give them your business). I use First American Title, but i'm not sure if they are in NY (I cannot imagine them not to be though). From this list, you can also see what they are actually buying. But call them up and ask them. Try to have some other deals from that area in your pocket when you do call them, even if they are not your own.
Unless you are in a rapidly appreciating market, the general rule is:
ARV *.65 - Repair costs - your profit.
The more competitive the area the higher that percentage is. Go to a REIA and ask around. In your area, it might be 100% of after repair value - costs because in 6 months the value in that neighborhood are expected to increase 30%. In that case it might be ARV * 1.0 - Repair costs - your profit.
This is learned from spending time in the market. A good realtor can let you know, but calling the buyers directly I think is the best.
One last hint, learn to create a good professional scope of work. This adds huge value to you are a wholesaler. It is a lot of work, but if you are good at it, serious investors will appreciate it and you will sell properties faster. And once you get a reputation, and your scopes are spot on, they will start to make purchases based off your recommendations. But that is about relationship billing and takes time to develop.
Good luck
Phil Cutting