Before I started any of the steps below, I'd be all over the seller and eBay like stink on **** for selling me a modified device. Reread the eBay posting though, to make sure that you didn't miss anything in the write up that discusses custom firmware. Maybe contacting them for a replacement or refund would be a good place to start. Remember that bad feedback can be a powerful tool. Contact them only through eBay's resolution process though, to ensure that you and eBay have a record of all your communication with the seller. Test out eBay's and PayPal's guarantees. If that doesn't work out, and if you used a credit card for the purchase, check with your bank to see if you can file a charge dispute with them.
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Now, if you want to try to fix this yourself, remember that doing any of this will likely void any guarantee or support from the seller or eBay.
Often, killing the root is just part of the process. If the seller rooted the phone and loaded a custom or modified Samsung OS, then the phone won't boot until that OS is replaced with the factory version.
Samsung's SmartSwitch has the ability to download and install the correct factory ROM for your device, but I don't know if it will work in your instance since you're coming from being rooted. It costs nothing to try though, and your phone is already bricked. Grab it at
Samsung Smart Switch™ . Once it loads, connect your phone to your PC and click on the oval button in the upper right corner "MORE". Make sure that the EMERGENCY SOFTWARE RECOVERY tab is selected, and hit OK then follow the prompts.
If that doesn't bring your phone back to life, you'll have to reflash the factory OS with ODIN. I'm not well-versed enough to walk you through it here, but there are folks at XDA who can help.
Good luck.