As long as I am paid, I do not worry too much about a response from the buyer. I also look at what they have to say in feedback they leave. Sometimes buyers are inundated with emails (especially buyers that purchase many items) and do not take time to read or reply to ones they consider superfluous. I think they consider the sale is verified when they pay.
I treat it as going shopping...you go to the store, pay for the item and go home. You normally don't call the store back up and say how happy you are but you do call if there was a problem.
Some selling sites try to make the whole buying/selling thing more of a social experience and some people are just not into that. Be happy you sold it, be happy you got paid, and be happy positive feedback was left. Don't worry or be irritated, Be happy.
JMO.