Are you sue that 'unique and used items' is the core of ebay? I have a feeling it's not anymore.
Plus with categories and search, I don't think they'll be drowned out. It's not a physical store - there's no space shortage.
Maybe not drowned out, but from what sellers tell me, there's a good chance that low volume sellers just won't be able to afford to continue selling on eBay because of the new fee changes that favor BIN pricing. Those are the types of sellers who generally sell 'unique and used' items -- so variety may indeed decrease on eBay as a result of these changes.
It's possible you're right, though I honestly don't understand why anyone would buy something through eBay when they can get an identical item through Amazon for the same price without the hassle and risk of fraud.