You have an addon in the marketplace and it hasn't sold. Some thoughts on why it hasn't sold and what you can do about it.

Can customers find it?

If no-one can find it, they definitely won't be able to buy it. The marketplace search is very basic and only uses titles, tags and categories. There is nothing like the sophisticated analysis of content and keywords that a Google search uses. Anything a potential customer may want to search or filter on needs to be covered by one or more of on title, tags and categories.

Can customers understand it?

Customers need to easily understand what your addon does or how it benefits them. A marketplace description and page should begin by clearly explaining its purpose and benefits. The page and documentation can then go on to fill in details.

Avoid excessive marketing hype; customers need substance, not just hyperbole. At the other extreme, don't dive straight into deep technical details. Developers often love detail, but not all customers need an intricate breakdown.

Can customers use it?

This doesn't become an absolute until after purchase. Nevertheless, before purchase a customer needs to be convinced they will be able to use an addon. Documentation and videos can help them understand how an addon fits in with their site and how they would use it. Built in documentation only helps after the sale.

Does it meet a real need?

An addon might feel essential to its developer, but does it solve a genuine problem or fulfill a specific need for customers? Addons born out of actual customer requirements tend to be more successful than those created as a clever idea or experiment. You already know that at least one customer needed it!

Competition

If there are many addons with competing functionality, sites requiring that functionality will split their business. The main differentiations are price and functionality.

• If there is a free addon, most customers will try that first
• If your addon is disproportionately expensive, most customers will buy one of the others.

If your addon offers better functionality, it has to be better from a user's point of view, not just a developers point of view.

Does it add value?

In most cases, wrapping up a 3rd party script will get some free downloads, but if an addon doesn't add value, it is unlikely to sell as a paid addon. 

Also consider many site builders looking for such functionality will tend to create their own block in preference to buying yours. By adding functional value, you give theme more reason to purchase.

Seasonality

Marketplace sales rise and fall with the seasons. Sales generally slump over the summer holiday season then pick up or even compensate in the Autumn. Similar patterns on a smaller scale surround the Xmas holiday.

Reputation

Customers are not just buying your addon or theme. They are buying your reputation. A positive and visible reputation will help you sell. A negative reputation can be disastrous. It can take years to build a reputation, but only minutes to destroy it.

20-20 Hindsight

20% of my addons account for 80% of my sales. Expect similar.