If you don’t have a lot of money to invest in employees and you’d rather work on your domain portfolio by yourself, you’d need plenty of free time. Generally, mini sites with 5 to 10 pages of content can be built within 5 days, while authority sites with over 10 pages could take up to two weeks. This amount of time would depend on your writing skills and knowledge of SEO techniques.
Above you provided great information about buying and selling domain names. I have been to many blogs about the same and I have read the people make millions just by selling domain names. It's a process that one needs to learn with time and can give a good earning by reselling the domain name. The business is fruitful but one needs to read, understand the value of a domain name. Buying every domain name is a waste of money. One must see the future potential growth for that particular domain.
The domain name industry is quite similar to the real estate industry in a lot of aspects. There are end users, brokers, consultants and domain flippers or “domainers”. Domain flipping works similarly to buying a house, renovating it (or even sometimes just sitting on it) and then selling it again at a higher price point. The gist of it is: you’re purchasing a domain name and betting it’s worth (or will be worth) more than you paid for it. If you’re right, you get a nice paycheck and move on. Those who make a living out of this just rinse, repeat, and scale.