Is it allowed to sell my plugin ?

Hi, i’m currently writing my own wordpress plugin; in which I put a lot, lot of time.

I have a little question about Licenses, etc. :

Is it allowed to sell a plugin you made ?

My idea was to show the plugin on a demo site; with a link to make donations and a progress bar (like for Wikipedia donations).

Once the progress bar would be complete, I would release the plugin under GPL…

What do you think of that and, most of all, can I do it ?

Thanks