do all proxies you buy give u a password and username? what if they dont

You don't always need a username/password, only if the proxy requires 'authentication'.

Some proxies that you buy authenciate you by username/password and some authenciate you by your IP-adress

Let's say you want to use VPServer as Proxy. You just need to set all your traffic to bounce off the VPS. This requires only authentication so that your PC can perform the action required.

 

In most cases, when you buy a pre-set proxy off a proxy seller, you will be given user name and password.

yeah if you don't get given a username and password i imagine its quite easy for someone else to find / use it