Make pure-ftp only accept virtual users

I installed pure-ftpd on Debian (Raspberry Pi), found the config file and managed to do basic configuration, disallow anonymous login, create a virtual user, call pure-pw mkdb and restart the server. But I can't login with this user. Instead I can login with a regular Linux user. I would like to reject regular users and only accept virtual users. I found that I should start the server with the -l parameter that specifies which logins should be accepted. This sounds great - if only I could find where the server is started. There is nothing in init.d and other known (to me) places that do such automatic daemon starts. As far as I know I did nothing to activate the daemon, this must have happened at the installation process. If someone could tell me where pure-ftpd is started, perhaps this would solve both of my problems. I wonder why a lot of stuff can be configured in the config file but such basic configuration like which users to accept must be set in a completely different way. What is the reason? Is pure-ftpd not the best choice for an ftp server that only should accept dedicated users with no relation to system users? #### Why FTP? I want to transfer images from my smartphone to the Raspberry Pi in the internal network, not accessible from outside. So FTP seems to be a good choice, especially because I expect a server that is described as "designed for security" to provide encrypted transfers too. With original configuration it seems to be open to every Linux user which makes me doubt this security a bit. #### Why this FTP server? I tried other FTP servers too but I didn't like they are tied to normal users. I don't want to add system users and assign home directories to them. I simply want to add two FTP users that can access one directory and save files as the user of this directory. I am not (yet) sure if this works, I'm going step by step and I am hanging at the virtual users for now. Documentation is not hard to find but most appears incomplete to me. In this case they all tell how to add a virtual user, but I had to search a while until I found I have to activate that. But no comment about *where* to activate that.