Magento 2 Cart – Safely Disable Add-To-Cart Buttons

I have used a few:

Aheadworks Ajax Cart Pro:

http://ecommerce.aheadworks.com/magento-extensions/ajax-cart-pro.html

Which works really well. You can see it in action on http://shop.harpersbazaar.com

Amasty Ajax Cart:

http://amasty.com/ajax-shopping-cart.html

Works pretty well, too. Big upside on this one is the javascript is entirely Prototype-based. No duplicated jQuery dependency to worry about.

But - don't be afraid to roll your own! It's not so difficult:

• The controller action needs to rewrite Mage_Checkout_CartController, specifically the addAction -- better yet, use a postdispatch event observer.
• The UI modal components for the success message are always on the page, so your ajax callback really only needs a true or false to display the appropriate message.
• Don't forget to provide support for related products (which can be added in tandem) and for various product types: simple, configurable, bundle. Don't forget grouped or downloadable, either. Everyone forget those :(

Best of luck!

If you are unable to save the products into a "special" category as @Julien suggested, you could set a different attribute set specifically for the products, or, if you can do neither of those, I would suggest setting a product attribute. Any way you do it, you'll still have to query the DB for these products, then grab your Cart and see if any items are in it.

So, let's create a product attribute, and call it "featured". Make a new attribute, as a Yes/No and assign it to the appropriate attribute set. Flag those 4 products as such.

Next, you need something along the lines of:

$collection = Mage::getModel('catalog/product')->getCollection();
$collection->addFieldToFilter(array(array('attribute'=>'featured','eq'=>'1'));

$exemptProducts = array();
$exemptProductFound = false;

foreach ($collection as $product) {
    $exemptProducts[] = $product->getId();
}

// now let's check if any are in the basket
$cart = new Mage_Checkout_Model_Cart();
$cart->init();

foreach ($cart->getItems() as $item) {
    if(in_array($item->getProductId(), $exemptProducts)) {
        // to make it simple, just set a flag
        $exemptProductFound = true;
    }
}

This could be in-line in your template, or in a function that you make available to your template (cleaner).