You can show current sales with a Product Discounts Widget.
To place a widget please go to WordPress widgets page (Appearance > Widgets) and find a Product Discounts widget on the left. Than drag and drop into the desired sidebar.
The Product Discounts widget has an option to exclude certain products by ID. When you set the products to exclude hit save and watch how it displays current sales in the sidebar.
This is how it looks in WordPress default Twenty Thirteen theme:
It comes with a predefined design which can be a little bit different in every theme. This is because it was designed too match every theme design. It can be easily minipulated with theme CSS.
Since version 1.5.17 you can also show a product image by checking the image checkbox inside the widget.