In order to add button(s) to the sidebar you will need to use one of the PHP function calls. Please refer to the WP eStore Shortcodes for a list of available functions:
http://www.tipsandtricks-hq.com/ecommerce/wp-estore-shortcodes-and-functions-reference-460
Now if you are calling the function from the theme’s template file (sidebar.php, header.php, footer.php etc) then you are fine but there is another way to do this using the widgets which could be more practical in some cases.
1. Download and activate a PHP executable plugin (there are many)
a) Executable PHP code plugin - http://wordpress.org/extend/plugins/php-code-widget/
b) Widgetify - http://wordpress.org/extend/plugins/php-widgetify/
c) You can find many more plugins here: http://wordpress.org/extend/plugins/search.php?q=PHP+executable&sort=
if you don’t have it already. These plugins lets you call php functions from the widgets (it’s similar to the Text widget but it lets you execute PHP code too).
2. Go to the widgets menu and add a ‘PHP Code’ widget to the sidebar (where you want to display the "Add to Cart" buttons). Now you can enter PHP code here to display the button for example:
<?php echo print_eStore_buy_now_button(1); ?>
This will display a ‘Buy Now’ type button for the product with product id 1.