I run Multisite and Marketpress.
At my subdomains (i.e. http://site1.mysite.com) my shop buttons show as "Add To Cart", which, when clicked, adds the item directly into the cart.
Over at my flagship shop (http://mysite.com) my buttons show as "Buy Now". When they're clicked you're brought to the product page instead.
Someone opened a thread about this here: https://premium.wpmudev.org/forums/topic/how-to-add-add-to-cart-button-on-items-list-instead-of-buy-now but a solution wasn't given.
Under Store Settings of my flagship store >> Presentation tab both "Single Product Settings" and "Product List Settings" have "Checkout Button Type" as "Add To Cart" button. Yet somehow it's not working.