I'm a merchant, can I use Squarespace to sell things?
While Squarespace does not have a built in shopping cart feature, you can easily link in PayPal links in order to sell items from within your Squarespace account. More information on that is available from Paypal directly.
You can use any third-party service to collect payments on your site that provides an HTML embed, such as E-junkie, Mals E-commerce, Foxycart, Google Checkout or BigCartel.
You can add the code to embed a payment button into any page of your site that's configured to accept code. In the WYSIWYG editor click the cog icon and add paste the code for your payment button there.
Note: Squarespace does not provide SSL for third party merchant services you've included in your site. You'll need to contact your merchant service provider directly for more information about their payment processing.