I used a free trial of Parallels Desktop 11 on my 2009 MacBook and the trial expired before I deleted my files from Windows. I'm going to restore the computer to factory settings before selling it using "clover leaf + R". But if the buyer installs Parallels Desktop will they get "my" Parallels Desktop (with all my files) because it's a virtual drive unaffected by system restore? If so, how can I erase my files from Parallels Desktop as well?
A "virtual" drive that Parallels creates when you make a Windows virtual machine is like any other file in your Macbook. Once you restore to factory settings (reformat) your old files (and the Windows virtual drive) are no longer accessible (not easily at least, unless they purposely try to recover your files using special tools). So you don't have to worry too much about your virtual drive unless you have very sensitive information there and you think the buyer will try to recover your files with recovery tools.
If you want to be extra safe, when you restore your computer and it is time to Erase your hard disk, click on Security Options and slide the slider all the way to the right to "Most Secure". This process will take some time depending on how large your hard disk is but it will ensure that the buyer will not be able to recover any files.
For more info, read this article and pay attention to step 4: https://support.apple.com/kb/PH22241?locale=en_US