It depends. There's no one right answer. If you're an experienced seller on eBay or Craigslist, you will likely get the highest value for your iPad on those platforms. However, they do require more effort from the seller 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 directly with your carrier or with Apple. These typically are the lowest payouts and may come in a form of a gift card.
Some older iPads, especially non-functional ones, do not have much of a resale value and you can choose to recycle them. Recycling in a environmentally friendly manner helps keep toxic pollutants, such as lead, out of the environment.