I've invented a new product and started selling it (in very small quantities). No one else is selling this product. Can I apply for a patent for it?
In the U.S. there is a bar against patenting anything that has been sold or even offered for sale that is 1 year after the first sale. Even if it is only one. If you want to patent outside the U.S. it is worse. In most locations it is too late to apply for a patent the day after you sell the first one.
To build on George's answer, if you have a new product, no doubt it will be improved as you get customer feedback. Those improvements may be patentable even if you have been selling your product for more than a year.