In most (if not all) countries/regions, Apple’s trade-in programme doesn’t accept current models of iPad. For current models, you may need to consider selling privately. You can verify current eligibility for trade-in and an estimated value here:
Apple Trade In - Apple
You should always consider the merits of product customisation with engraving - as this may impact future values, in particular if attempting to sell privately.
If accepted for trade-in, an Apple-engraved device may be accepted with little if any impact upon the trade-in value - which will always be considerably lower than the price that you paid.