I believe his logic was that he went to the purchaser's bank to confirm that the the funds were available in the guy's checking account right then. He then decided to cash the check there to be sure that the guy didn't draw down the account before the check cleared, i.e. make a few ATM withdrawals which would later cause the check to bounce at his credit union.
Maybe so, but why didn't he just get a cashiers check. Why would anyone want to carry that amount of cash just to redeposit it.