If you can reply and ask for a phone number, then chat with them, you will get a Much better idea of there interest in the domain name.
Just remember 9 times of 10, people are just looking a deal, so no matter what you reply with, they are going to just not reply or reply back with something close to the first offer.
#1, it's hard to get an offer
#2, 9 X's out of 10, when you get an offer it's a Bargain Hunter
#3, Don't worry about it when somebody doesn't reply, it happens
#4, Stick with your asking price, if they want the domain, they will pay