Seems the easiest thing would be to edit the product.tpl and products.tpl to remove the add to cart button if you didn't want to sell anything. If you wanted to just sell certain products you could add a new availability field to the products table of the database, and then add something like this to the templates.
Code:
{if product.availability eq "Y"}
--add the add to cart button code--
{/if}
Should be pretty easy to do. I could go more in depth if anyone is interested.