either way, i personally just buy the blank ones, but you can refill the used prefills. check the link on the post above, and check out the section in the modders forum for "cart mods" theres a lot of great mods in there that can save a lot of money on carts. i use the auto drip mod (ADM, by scottyballs) and i have been using the same cart for over a month now. its basically just an empty cart with a spring and a polyfill, or some other fill just used as a plug to keep the liquid in the cart, they work great!!!