You are alarmed if the previous owner can access all your voice request but not the NSA?
To answer your question, it's unlikely the previous owner can. But it is highly likely the NSA can.
Just saying.
Are you serious?
Do you think the NSA have nothing better to do than to act on audio they have heard through Echos?
If someone is overheard plotting an act of terrorism do you think that is wrong that the NSA should take an interest?
If they heard you saying you got into the cinema for free I don't think a SWAT team would be at your door.
Personally I WOULD be more concerned about someone I knew possibly having access to my voice commands more so than a government body.
If my order of Viagra was delivered to my next door neighbour in error I would be very embarrassed if they opened it. On the other hand, if the NSA knew I had bought it, WHO CARES.