I know a guy buy retail stores, then sell it after few days. Domain flipping seem to be the same thing. Only you buy a domain/website(have make a website if there is no website), then make some improvements and wait to sell. While you are waiting to sell it you can earn some money from ad revenues or so.
I don't have first hand experience on this but I think you would need a server to host multiple websites, some html coding knowledge etc. And of course you have to know what to buy.