Having done this for a long time now, the most common mistake I see by far is where a page does a great job of leading the reader into a buying mood and then drops them like a stone.

Every site needs a "Call to Action".

A Call to Action is exactly what it sounds like: the opportunity for the buyer to buy, or the reader to take the next step.

It's painfully simple: you need a "Buy Here" button, or a "Click here for more information" link. Anything that doesn't leave the reader searching the page for something to do. An effective call to action is by means of an internal link. This is often placed within the text and will lead to a detailed description and a Buy Now button. Typically it would look like this:

"Kitchen waste disposal units are underused in the UK, compared to the amount in use in the United States. One of the most popular makes is Tweeny, which provides a range to suit all budgets"

Now the word "Tweeny" links through to your product page, showing the available range and buying details, where the "all budgets" might go either to exactly the same page, or just to the pricing section. Internal links are good for SEO and helpful for the reader.