While eBay's default metadata for listings cannot be changed, you can use 3Dsellers to utilize the <iframe> that contains your item's description (shown as a separate page on eBay mobile).
Custom mobile description metadata is also inserted in the description. This causes eBay to override its own metadata for a mobile description.
Image Alt Tags
Image Alt tags are only available for images that are included in your eBay description (or design template).
eBay automatically applies the listing's title as alt tags for product images, which cannot be edited.