HTML & CSS template sellable?

Hey there,

I am a webdeveloper, but I’m not very familiar with PHP,
and I just started to learn using WordPress.

I used a HTML to WordPress converter, which works perfectly.
That means, I make a full website in HTML and CSS, and then I seperate it and turn it into a WordPress website.

Now, I was wondering, if I make a theme, which is basicly PHP containing the HTML-code, into a theme and sell the theme?
Or will it make difficulties for people who would want to add plug-ins etc?

Thanks ??