It depends. There's no one right answer. However, MacBooks hold their value very well and if you're an experienced seller on eBay or Craigslist, you will likely get the highest value for your MacBook on those platforms. However, they do require more effort from the seller, charge fees, and come with risk of fraud that novice sellers may not know how to protect themselves from.
For those customers, we recommend using a trade-in site such as ItsWorthMore. Trade-in websites leverage their volume discounts and industry know-how to provide a smooth streamlined experience; for a slightly lower payout.
Another alternative is to trade-in your MacBook directly with your big-box retailers or with Apple. These typically offer lowest payouts and often come in a form of a gift card.
Some older MacBooks especially non-functional ones, do not have much of a resale value and you can choose to recycle them. Recycling in an environmentally friendly manner helps keep toxic pollutants, such as lead, out of the environment. Keeping us all healthier and happier!