For any vendor you are considering using, you should determine how long they have been in business before you make your purchase. While time in business is not a perfect indicator of the quality or stability of a company, it's a useful data point that tells you something about the company's legitimacy.
Few companies stick around for years unless they are doing something right.
Where time in business becomes much more important is when dealing with a PC or component manufacturer . So for example, if a PC maker is offering a two-year warranty, then I would be look to see if they have been around for at least two years. In my experience, most companies will be honest about this because they know how easy it is to verify the figure, and they don't really figure they have any reason to lie. If you find a vendor that appears in all other respects to be very good but has not been in business for very long, you may still have a good experience with them. Many new companies are quite excellent, and everyone starts out new at one point.