Chip Bennett

It appears that the Spun Theme only supports dynamic sidebars in the footer. Also, it appears that the design of that Theme is such that footer content isn’t visible unless/until you hover over it.

Your footer sidebars are working fine.

Look on your site, beneath the post images, but above the footer. You should see a plus sign. Hover over it, and you’ll see it has a black, circular background. Click it, and your sidebars will display.

It appears that the Spun Theme only supports dynamic sidebars in the footer. Also, it appears that the design of that Theme is such that footer content isn’t visible unless/until you hover over it.

Go to Dashboard -> Appearance -> Widgets. What Widgets do you have assigned to each of the three sidebars (Sidebar 1, Sidebar 2, Sidebar 3)?

That site is distributing Themes that are not GPL or GPL-compatible, that put spammy links in your footer, that mis-apply the Creative Commons Attribution license clause to attempt to force you to display spam links in your footer, and that include encrypted kill code that prevents you from modifying the spam links in your footer.

Such sites should be avoided at all costs.

Get a better Theme here:
http://wordpress.org/extend/themes

…and the entire site went blank white, no errors just blank.

Enable WP-DEBUG, and report back the fatal error(s).

See also:
http://codex.wordpress.org/Debugging_in_WordPress

Well, if you insist on using template_redirect:

function my_template() {
    if ( is_single() && in_category( get_cat_id( 'for-sale' ) ) ) {
        include ( get_template_directory() . '/salesposts.php' );
        exit;
    }
}
add_action('template_redirect', 'my_template');

The width of the single column. That’s the same width as the embedded YouTube clip which is 700.

Okay, I’ll assume that you don’t care about height, but only width.

In that case, add the following to functions.php:

function stevenstallone_add_custom_image_size() {
    add_image_size( 'post-title', 700, 9999, false );
}
add_action( 'after_setup_theme', 'stevenstallone_add_custom_image_size' );

That function will register a custom, featured-image size. The 700 is the width, in pixels. The 9999 is height, in pixels. The false indicates that the cropping will be a “soft” crop, i.e. “box resize”: it will resize the image using the constraining dimension. So, if you have an image that is 1400x200px, it will be resized to 700x100px.

But, just registering that size isn’t enough, as any already-uploaded images won’t have that size generated. So, you’ll need to regenerate thumbnails, e.g. via the Regenerate Thumbnails Plugin.

That Plugin will force WordPress to regenerate intermediate image sizes for all uploaded images. Afterward, you’ll be able to reference/use your custom image size.

Next, you need to modify the template. You were on the right track before:

<h2>
<?php
if ( has_post_thumbnail() ) {
    the_post_thumbnail( 'post-title' );
} else {
    the_title();
}
?>
</h2>

I’m looking for the loop that only shows the post of that category that is chosen, nothing to do with template definitions.

WordPress does this automatically. For example, for category foobar, the following URL will display all posts with the foobar category:

www.example.com/category/foobar

WordPress will use the category.php template file to render this page. If you want to override the display for only the foobar category, crate a template file named category-foobar.php. (More information: Template Hierarchy: Category Display)

If that doesn’t suit your needs, please provide more detail regarding what exactly you’re trying to accomplish.

What (in human terms) are you wanting to display with this template file?