Thanks Marco, it's interesting to hear that the sellers have other options to use as the reason for cancellation, including one that fits my situation better than the one they chose, but also a relief that it won't affect my account anyway!

Thanks for the link. I won't report the seller, as it seemed to be a genuine error on their part (that they couldn't find the item). They were perfectly pleasant about it and didn't delay in proactively refunding me everything I'd spent (the same applies to the other seller in the earlier situation). So no lasting harm done to me aside from a temporary disappointment in not getting the item I'd thought I'd bought.