There seems to be an unasked question here which is how to not only uninstall Ubuntu, but how to prepare your device to be resold to a third party.
Formatting the hard drive is not enough to prevent someone from stealing your personal data.. That's because formatting does not actually delete any data.
If you are selling the device on craigslist you should securely erase all of the hard drives on the device. SSDs usually have a "secure erase" feature which will allow you to quickly and securely wipe the whole disk. SSDs without secure erase and all HDDs will need to have the entire disk overwritten using a tool like DBAN.
If you want to sell the device with an operating system preinstalled, you should cleanly install the OS only after all disks have been securely erased, without inputting any personal or identifying information.
If you sell the laptop on craigslist without removing all operating systems you used and without securely erasing all the storage, you may live to regret it: identity theft is very real and increasingly common. One of the easiest ways someone can steal your info is obtaining hardware that was not securely erased.
