Using the internet to buy fabric is a wonderful option if:
a) you don't live near a quilt store
b) your quilt store does not have exactly what you are looking for
c) you need more of a specific pattern of fabric and your quilt store has also run out!
But there are two big things to be aware of:
a) Buying on-line means that you (almost always) will pay shipping fees. So this adds to the cost of your fabric. Since these fees are often based on weight, you might want to buy additional fabric (which you will certainly use for something) and not incur any additional shipping fees.
b) The on-line pictures of the fabric swatches are not always 100% accurate! If you are buying a fabric that you know (for example, if you need more of something), then there should be no problem (other than a possible dye-lot issue). But if you are buying sight-unseen and trying to match other fabrics, what you see is not always what you get.