The most common issue with the Sell Content add-on is the misconception of what the add-on actually does. The add-on sells access to the content field and not the entire post/page. This means that certain post types with certain plugins will not work.
An example of this is selling access to WooCommerce products. While you could sell the content for products, it will become useless since the content is only used for the product description. Everything else about the product is presented outside the content field and hence would be fully visible.
Some themes uses the content field to render parts of the websites design. This in turn means that when the content is set for sale and is blocked from loading (since the user has not paid) required files or features will fail. This issue will present itself as a visual issue where the page loads incorrectly or not at all (depending on the theme / plugin).
As an example, if your theme requires that Visual Composer is loaded on every single page (even on pages that are set for sale) then the websites design will be affected if viewed by someone who has not paid for the content.
There is no simple solutions here since two features are directly colliding. This unfortunately means that if your theme requires unfiltered access to the content field, the add-on will cause visual issues when enabled. In these situations you will need to try and find a custom solution.
As of version 1.7.6 you can use the mycred_sell_content_priority filter to adjust the priority for the content filter. You can use this if you need to move the content filter in order to let other plugins / themes / custom code snippets to have a chance to run before the content is filtered out.