Try this post:
http://www.markettheme.com/forum/support-area/removal-of-search-bar-and-sidebar-mkt-401

A great tool I use for CSS changes, is a plugin for Firefox named "Firebug".  It's a free download, and I highly recommend it.

The background color for the margin is pulled from the "/imgs/" folder.  It's a file named "cont-bg.jpg".  As far as the style element, I believe it's "#main .content". 

Obviously, on the second link above you'll make the margin smaller – not wider.

Ok, here's again how to completely remove the sidebar, so that your content is the entire width of the page.

These code changes are for version 4 and up.

1.  First, edit your "sidebar.php" code. 

Enter this at the very TOP first line of the file:

<?php /* ?>

and enter this at the very BOTTOM of the file, as the last line:

<?php */ ?>

These lines are different if you look closely, so make sure you've got them correctly.

Save the file.

2.  Now, open up your "functions.php" file for editing.

On or around line #63 you'll find this line:

$mkt_options['mkt_body_content_width'] = $mkt_options['mkt_TOTAL_PAGE_WIDTH']-270-65; // Total width minus left column width, minus content padding

You'll need to change it to this:

$mkt_options['mkt_body_content_width'] = $mkt_options['mkt_TOTAL_PAGE_WIDTH']-35; // Total width minus content padding

3.  Next, on or around line #79 you'll find this line:

#main .content .newproducts .new .itemdesc { width: <?php echo $mkt_options['mkt_body_content_width']-$mkt_options['mkt_thumbnail_width']-30; ?>px; // Minus left column width, minus content padding, minus thumbnail}

You'll need to change it to this:

#main .content .newproducts .new .itemdesc { width: <?php echo $mkt_options['mkt_TOTAL_PAGE_WIDTH']-35; ?>px; // minus content padding}

Save the file.

That should do it.

No, you can actually use the built-in WordPress Theme editor tool. It's in the admin area under the "Appearance" menu tab. In there, just click on "Editor", and you can edit many of the Market Theme files without needing to FTP.