Thanks for your input, everyone - it's much appreciated
To be clear, my intention wouldn't be to shame the seller or be vindictive in any way, but merely to warn others.
It
seems more than a coincidence that right after my lens arrived back with them, and my refund was received, one copy of the lens was suddenly back up for sale on eBay - despite being out of stock since I had bought mine. That said, I can't be certain this is the same lens, and since I have no reason to be personally dissatisfied with the seller (given the prompt refund), I'll err on the side of caution and let the matter rest.
I do know I won't be buying from that seller in future, though, despite their tempting prices!
Thanks again, folks