Understanding 'used' term

[2swanz]

One of buyers wants to get return because his item was turn on. I selling a tablet, which is in perfect condition and seriously never been used.  I'm selling it because box are damage, items inside missing....etc. Of course I add option 'open - never used'. The most important I need to check what I'm selling so I turn to get information like serial number or IMEI. 

I'm wondering because I working for one of most popular retail shop... we open a phone and add inside our Sim Card and later selling as 'brand new', sometimes add our apps and its call 'development' and still sell as 'brand new'. 

[Anonymous]

If items are missing, it's not new

• New: The brand-new, unused, unopened, undamaged item is in its original packaging (where packaging is applicable). Packaging should be the same as what is found in a retail store, unless the item is handmade or was packaged by the manufacturer in non-retail packaging, such as an unprinted box or plastic bag. See the seller's listing for full details.
• Open box: The item is in excellent, new condition with no wear. The item may be missing the original packaging or protective wrapping, or it may be in the original packaging but not sealed. The item includes original accessories. The item may be a factory second. See the seller's listing for full details and a description.
• Certified refurbished: The item is in a pristine, like-new condition. It has been professionally inspected, cleaned, and refurbished by the manufacturer or a manufacturer-approved vendor to meet manufacturer specifications. The item will be in new packaging with original or new accessories. See the seller's listing for full details.
• Seller refurbished: The item has been restored to working order by the eBay seller or a third party. This means the item was inspected, cleaned, and repaired to full working order and is in excellent condition. This item may or may not be in original packaging. See seller's listing for full details.
• Used: The item was previously used. The item may have some signs of cosmetic wear, but it is fully operational and functions as intended. This item may be a floor model or store return that was used. See the seller's listing for full details and a description of any imperfections.
• For parts or not working: The item does not function as intended and is not fully operational. This includes items that are defective in ways that render them difficult to use, items that require service or repair, or items missing essential components. See the seller's listing for full details.