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You could also check if you get any trade-in value from Apple. Wether or not it’s worth to sell it is a personal preference in my opinion. Only you can decide what you need. Personally I always try to get rid of things I don’t use anymore.
 
I would say so - you'll still get something for it if it is in reasonable shape. Better to do it sooner rather than later too as resale prices will probably drop a fair bit when iOS 13 is announced at WWDC in June 2019 (that is assuming that A7 devices, including the iPad Air will not receive iOS 13 - if they do receive iOS 13 that changes the story somewhat).
 
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