My original iPad has seen better days. Its screen is all marred, scratches and dents adorn its back shell, and iOS 5--yep, you read that right--creaks along on its outmoded internals. It's time for an upgrade. If you're in the same boat as me, take heart: You don't have to be stuck with a tattered slab of aluminum and glass after you upgrade.Lots of services allow you to sell back your old iPad in exchange for cash or store credit--or at least unload it on someone else who will dispose of it or recycle it for you. Here's what you need to know about four popular gadget-buyback services, and how much they'll give you for that "ancient" 18-month-old iPad you can't bear to use anymore.AmazonAmazon will take back your old iPad in return for an Amazon gift card through its electronics trade-in program. To use it, visit Amazon.com, and click Shop by Department in the upper-left corner of the page. Click Electronics & Comput
