The main selling point of a older used computer would be the CPU and the original operating system disk to go with it., and possibly the ram, but ram is now cheap. If a computer has for example a 800 MHz or 1.8 GHz cpu well that's not really going to draw much interest.
A computer could have photo editing software, etc. worth several hundred dollars. But a potential buyer may not have any use for the software.
1) Selling with the software installed is fine, but one must include the install discs or the install and use of then becomes illegal. In most all cases, selling pre-installed software with the discs makes for a legal transfer of licence.
If the software is downloaded software, the licence becomes more difficult to transfer, sometimes impossible, as this software is generally tied to a particular user and that users email address.
2) Yes, OS discs are the same as other software. You may sell the discs to a new user and this transfers the licence for use of that software.
A note regarding both of the above questions, one may find that the software alone is worth more than in a package with a machine that someone may or may not want.
This is definitely so with a Leopard install disc. That disc, alone, can often fetch more than most G5 machines are worth.
In combination with the machine, the actual net proceeds may be less than if sold seperately.
3) Creating a new User account with Administration privileges, followed by deletion of the old user account will "remove" the old user account and data but leave the software. The data is actually still there, until overwritten, but is not directly accessible unless data recovery software is used.
This method is not a secure method.
If you wish a secure deletion of personal data, erase and write zeros before reinstalling the OS.