I sold a rifle once and asked for a USPS money order and I was sent a Wells Fargo cashiers check.
I was a little put off by the fact that my request was ignored, but not wanting to lose the sale, I contacted Wells Fargo and asked them to trace the cashiers check and confirm its validity.
A little online research and one phone call to Wells confirmed that it was a legit check, purchased by the buyer and without a "stop payment" on it. I deposited it my bank and mailed the rifle to the buyer.
I'm sure your seller could do the same thing if he chose to. He's just playing it safe by waiting for it to clear.