ECommerce is a new module and it is still not very customizable. You can add the button to the List template of all products, but it wouldn't work because right now the productID cannot be resolved unless you have navigated to the details page of a selected product. This said, for now you will have to click on a product (to open the detail page) and then add the product to your cart.
This feature is logged into our system and is now on our roadmap. However, we currently have a lot of features that are set up for our next release (4.3), so this will be released as an option a little further in time (possibly in our 4.4 release).
Editing the templates wouldn't solve the ID problem. The only way would be to implement a custom control but it would have to re-implement the whole list functionality there (to change the way the productID is resolved).
I'm sending you some examples on how to use the ECommerce's API to create Customers, add them Orders, create Products and so on. Please review the examples and get back to me if you have any questions.
You can try using the Telerik.Sitefinity.Modules.Ecommerce.Orders.Web.UI.AddToCartWidget in your template and in your code get the control and set its ProductId property to the selected product in your list (this way overriding its default functionality by getting the productId from the URL)
You can always change the template of your control. What you need to do is - you need to inherit the AddToCartWidget class, change the LayoutTemplatePath property (in order to change the template). At initialize control, the Text property of the QuantityControl (that is the TextBox on the template) is always set to "1" so if you just hide that field, then its value would always be "1". Here's the default template that you can change (for example add a css property to the TextBox that hides it):
<%@ Control Language="C#" %>
<%@ Register TagPrefix="sf" Namespace="Telerik.Sitefinity.Web.UI" Assembly="Telerik.Sitefinity" %>
Text='<%$Resources:OrdersResources, QuantityColon %>' AssociatedControlID="quantity" CssClass="sfTxtLbl" />
Text="<%$Resources:OrdersResources, ProductQuantityIsRequired %>" /></
Text="<%$Resources: OrdersResources, ProductQuantityIsInvalid %>" /></
Text='<%$Resources:OrdersResources, AddToCart %>' ValidationGroup="addToCart" CssClass="sfAddToCartBtn" />
Here is the list of steps that you need to do
Things to note in CustomProductMasterView.ascx file –
<%@ Register Namespace="Telerik.Sitefinity.Modules.Ecommerce.Catalog.Web.UI.Views" Assembly="Telerik.Sitefinity" TagPrefix="sf" %>
ProductId= '<%# Eval("Id") %>' ProductsListPageId ="B23147EE-D087-4147-8C13-147902562DA4" CheckoutPageId="6C588951-BBFA-4C70-A983-46A41DA3B022" LayoutTemplatePath="~/CustomBuyNowView.ascx" />