You can verify eligibility for trade-in and obtain an estimate of the potential trade-in value here:
Apple Trade In - Apple
You should be able to trade-in at a physical Apple Store - many choosing this option by preference as the device can be checked, the trade-in value finalised and the process completed while in-store. Be aware that Apple’s in-store trade-in values may differ from the on-line estimate.
Before parting with your iPad, whether trading-in or selling privately, ensure that the device is properly prepared:
What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support