Make a list of all the services you use (plus logins). I used a spreadsheet
Transfer ownership of all accounts to an email address you will had over to the new owner
Change the address in domain registrar and all service accounts (once sale is complete)
Use a (fairly) modern tech stack. My buyer was impressed that they could deploy using docker-compose and found the code easy to read and understand
Make sure all accounts use their own email address (do this from the start even if you never intend to sell, it will save you a headache later)
Use a broker (indiemaker which uses escrow.com) to cover yourself from being scammed (escrow will release the money once you both agree that the assets have been transferred).
don’t under sell yourself. It might feel like your product isn’t worth very much (the code is so crappy, or it’s only a bunch of if-statements). What purchaser is buying is more than the code. They are purchasing all the hours of work you’ve put into this product. If you add up every hour you’ve spent on it and even charged just minimum wage, I’m sure you’d come up with more than you were planning on charging.
Revenue is key. It doesn’t matter how clever your application is, how great the design or the code is. If you don’t have revenue then there’s a maximum you can charge for it (and you won’t be buying a helicopter any time soon). Revenue (it generally doesn’t matter how many customers that is from) shows that you have a product with an addressable market. That is absolutely worth something to the buyer.