There have been a handful of times in the 14 years I've been on
ebay that I've either accidentally relisted an item that previously sold or somewhere along the line lost the item that was listed (hey, it's life; crap does indeed happen occasionally).
Never on my worst day would I consciously ship a buyer something other than what they paid for. That's just plain wrong, no matter how you slice it.
I would contact the buyer, explain my mistake, apologize, and ask how they would like me to remedy it: full refund, a substitute similar item, a comparable different item from my inventory, whatever.
Just arbitrarily assuming what would please the buyer is presumptious. I tend to side with the comment above that this was deliberate and the seller was assuming the buyer wouldn't notice.
And I also agree that the substitute items are of subpar quality.