Look for vendors that have developed around the inflection point. Additionally, choosing the largest vendor may not make you safe. When selecting a vendor for e-commerce deployment, it is important to look beyond the life of the vendor. The CEO should assume that the vendor will not be with them in some way or another in five years. Software will change, the vendor may change focus, be bought out, or they may discontinue the product.
Technology has proven time and time again, even safe vendors can go away in five years.
This also might be a good time to re-evaluate your knowledge management procedures. If the vendor cannot supply the source code, look very carefully at them. Any vendor that cannot provide the source code to what will become the core of your business initiative on the web shouldn't be taken seriously. Remember that you are betting your business on this. Think about your relationship with your current IT vendors.