You create the landing pages from your personal account, you are responsible for configuring your client's personal links, maintenance, etc. The client would only be responsible for advertising by putting the link to the landing page you sold them. You can sell them for a year with the domain included, and every year, you charge for maintenance. You should also provide the service of making changes to the website, perhaps a couple of times a month or a total per year. Remember that optimization is also required for the landing pages, so if someone buys the service, you should provide website changes included in the original price