I can only really comment on securing content as I haven't used any ecommerce solutions with craft. But as far as that is concerned — one approach would be to add a related entries field-type on the users' field layout (called 'purchasedProducts'). When a user purchases a product you can add that entry to their purchased products via a plugin. You can then check the current entry against the related entries to determine if the user is authorized to view the content.
{% set authorized = false %}
{% if currentUser %}
{% if entry.id in currentUser.purchasedProducts.ids() %}
{% set authorized = true %}
{% endfor %}
{% endif %}
{% if not authorized %}
{% redirect 403 %}
// or display product teaser and purchase button
<h3>{{ product.title }}</h3>
<p>{{ product.description }}</p>
<a href="purchase/product/path?id={{ product.id }}">Purchase</a>
{% else %}
{# output entry #}
{% endif %}
To create an index page of all purchased entries, you can loop through the currentUsers purchased entries.
{% if currentUser %}
<ul>
{% for entry in currentUser.purchasedProducts %}
<li><a href="{{ entry.url }}">{{ entry.title }}</a></li>
{% else %}
<li>You have not purchased any products.</li>
{% endfor %}
</ul>
{% endif %}
To show all products and add a 'purchase' or 'view' button depending on whether they've purchased the product or not, you can check if the entry is in a 'purchasedProductsIds' array.
{% set purchasedProductsIds = [] %}
{% if currentUser %}
{% set purchasedProductsIds = currentUser.purchasedProducts.ids() %}
{% endif %}
<ul>
{% for product in craft.entries.section('products') %}
<li>
<h3>{{ product.title }}</h3>
<p>{{ product.description }}</p>
{% if product.id in purchasedProductsIds %}
<a href="{{ product.url}}">View</a>
{% else %}
<a href="purchase/product/path?id={{ product.id }}">Purchase</a>
{% endif %}
{% endfor %}
</ul>
Obviously you will also need to add a small plugin to respond to the purchase process (via a hook or event) and update the user's purchasedProducts. Check out usersService saveUser method for more info or search this site for many examples.
As far as ecommerce solutions, I haven't used any with craft so I can't really comment. However, I did find a tutorial on managing your craft inventory with snip cart, which should describe everything you need to create the webhook.
