If I sell the a template am I still allowed to use this template on my own website or sell it again to another person? Or is this something that has to be worked out with the buyer?
You probably want a clearly stated user license to go with your templates for sale. That would spell out whether it was an exclusive or non-exclusive sale, and what the buyer can do with it. Check out some sites that sell templates to see how others manage it.
When you have sold something. Isn't it just right for the client to have the rights for it? He have already paid you!
IF you are considering doing this then you obviously want to have something written up about other people purcashing it. What I have seen done is you have a common price say $150 and anyone can buy it. Then you make a unique price say $1500 (The same as selling 10 templates at common price) and you give people the option to purchase either one. The difference is that if someone purchases the unique version, you take it off your website and you no longer sell it to anyone else so that website will have its own "unique" template hence its name. Just an idea.