This happened to me with a card that I really wanted. Like the OP, the seller sent me a kind note saying he couldn't find it (???). However, when I expressed my disappointment that he "lost" the card and told him I really needed it for a set, he magically found the card again. To his credit, he had already refunded me, but shipped the card to me for free, which was a nice gesture.
I can't tell if he was more afraid of bad feedback if he didn't "find" it, or if he genuinely just lost it. Either way, it worked out happily in the end, but it is still pretty sus.
#COMMONCARDSMATTER
"The 0-2 pitch... SWING AND A MISS! Struck him out! The Philadelphia Phillies are 2008 World Champions of baseball!"
- RIP Harry Kalas