If you would like to find out how to build duckweed ponds, I have written an article on Hubpages.
Although we use it for feeding our tilapia, there are people who feed it to chickens and other animals. It is also consumed by people in Asia. Besides using it as a feed supplement, you can also make money from growing duckweed. If you take a look at Ebay for example there will be people selling it. They are also selling it on Amazon. When you realize how fast this plant grows you will understand that this could be a little gold mine.
Let's look at what you will need to do to sell your plants. Firstly you will need an Ebay account. If you are like most people you already have one. On you write up about it, tout the benefits of using duckweed. Remember it isn't just about farm animals. Think of all the people who have aquariums. Even in they grow it in a bowl on their kitchen counter for their goldfish, they are still a potential customer of yours.
***One caveat here is, you can't sell plants abroad. Check the restrictions in your country and with your postal service. Most countries do not allow this as the chance of transfering diseases and pests into a new environment is high. That said, their will be a market for it in your country.