I thought it was the other way round? That having your own site gives you control?
The plugins I cite work in the background and aren't visible and I don't think they verify what a person sells - like the WP eStore Plugin, you can't know if a site uses it especially if, like me, it has customised the "add to cart" button or the shopping cart page (I didn't bother to change much).
But I had not thought of age verification so can't say if they are an option in that regard.