Once you have built up a loyal twitter following, you CAN promote products, either your own via your web site or blog, or through affiliate links. The trick is to make sure that the products you want to promote and offer are related to your twitter content and that your followers can really benefit from the product.
The people who read your tweets should feel that you are more interested in their best interest, as opposed to your own, as you send them to consider the product you are recommending. Your twitter reputation is on the line with every offer your recommend, and if they seem consistently self-serving, you may see your twitter following, or at least your click through rates, drop off significantly.
On the other hand, if your products are continuously perceived as valuable to your followers, your traffic and hopefully your profits, will continue to build.
