Agree.. This is suspicious.. I don't believe this guy at all.
There's no reason why he couldn't take his check straight to his credit union.
He went to the buyer's bank to verify the funds. More than likely he did not have an account there, thus the $5 fee for cashing the check. Banks (and credit unions) will allow a non-member to cash a check if it's one of the banks' own checks.
It's not suspicious. He sold a trailer, and the check that was given was drawn on a different bank. He went to that bank to cash it, so that he would be able to find out immediately if the check was good. He then took the cash to his personal bank to deposit.
I have done this exact same thing on several occasions when selling vehicles.
"There's no reason why he couldn't take his check straight to his credit union. "
If he didn't know the buyer, or maybe if he did even, he certainly wanted to verify there were funds for the check.
Could be he was trying to avoid the 5-day float. Sounds like something I'd do in a similar circumstance.