What I would do is,

I would politely advise the buyer to contact the postal service that delivers to this "old" address on the check "he" mistakenly sent you. I would have him request that a flag be put on the missing package. If they're successful in tracking it down, everyone's happy. If they're not, then ask yourself, who's fault was it and who should bare responsibility.

If I had sent the wrong check and the chips never turned up, then I WOULD ASSUME RESPONSIBILITY. No questions asked.

But that's just me,

Sam