It is correct to enumerate a list using numbers in parenthesis after a colon. Here, since you do not have complete sentence before the colon, you will need to separate the items in your list with commas instead of periods and the first word of each item should be lower case: "This product is popular because: (1) it is inexpensive, (2) it has a two-year warranty, (3) it is made of environment-friendly material." You could also make the sentence more concise by using by using a more traditional list: "This product is popular because it is inexpensive, has a two-year warranty, and is made of environment-friendly material."