I have built a few things that I have sold, But NOT with the sole intentions of making money off of them like a manufacturing company does. Have sold three different lawn tractors that I bought for a little bit of nothing at garage sales and built front end loaders for them and had only about a 800 to 900 dollars tied up in them but sold them for 2000 dollars. Pretty good profit I would say, doubled my money in less then 3 months and have to say that 100% interest in 3 months is better than any investment into a bank CD or the stock market. Though there is a lot of labor involved. Also sold a Boom pole that I got all the material free and a drag harrow. I see nothing wrong with this it is somewhat easy money. I love to weld and tinker with things so why not make a little extra income while doing it to pay for my hobby. Some people don't know how to do this stuff so they just buy it already made. This is a great way to make a little extra money. I have sold everything using criagslist which is a supper terific place to advertise for free. good luck