There are two types of Add to Cart buttons in sellacious.
For these type of buttons you don't need to create any product in sellacious. You can directly create these buttons from Sellacious Backend.
To create add to cart buttons, go to sellacious Backend > Shop > Buy Now Buttons.
Click on New button on the right top of the page to create an Add to Cart Buttons.
The view will open to create sellacious buttons. Fill all the fields.
Now, to publiush this button on your site/frontend, you can either put this button in a Article or create a Custom HTML module.
Below is the image of this Add to cart button in custom HTML module.
In sellacious you can use buttons of already created products too. In Product List there's a column for Button Codes. You can copy Add to Cart button code from there and use it.
The code copied for add to cart button of the product will be like this
[sellacious.cart.add=P1V0S323;btn btn-success;Add to Cart]. Paste this code in Article or Module. And it would look like this.
In module make sure to select 'Prepare Content' to show the button.