Location
Location is clearly very important. The local greengrocer's that services a housing estate is less likely to survive these days as people tend to drive to the local supermarket for its cheapness and convenience, neglecting local businesses. This isn't going to change but a greengrocer's can make the most of its circustances by having a good location.
It is also worth noting that the fruit and vegetables market is a seasonal one. Consumers know that there are times of the year when certain products will cost more. Obviously, this will also affect you, the greegrocer, so think about time of year when buying your business. And don't forget that all the stock is perishable so you'll need to start buying in what you need straight away.
Be wary of buying a business that doesn't appear to be doing well, despite good market conditions. There are always other reasons (such as poor management) but a greengrocer's that has alienated its client base in spite of, for example, a good location, will not be worth much.