I've witnessed this knives policy of eBay's get exponentially more ridiculous over the last few years (when, back then, I attempted to bid on some butter knives but couldn't without a credit card)!
And the consensus seemed to be this is how eBay are choosing to deal with it, so you just have to lump it (and buy elsewhere if you don't own a credit card).
But if they are seriously banning things such as (round-edged) bookmarks, I do think it is spiralling out of control now.
Where does it end, eBay!