what does a flashing folder with a question mark mean

So you didn't follow the recommended method of erasing your hard drive? (See this article on the proper steps to take). What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

This article explains what's happened, and how to deal with it: If your Mac starts up to a question mark - Apple Support

Well, it sounds like you erased it all right... That means your Mac can't find a bootable volume.