Always reply! Unless your name is an OBVIOUS target for WIPO (I'm talking generics here, not squatting).
Your note should just be: "Thank you for your offer. However, we are not interested in selling." People think that not replying means you will yield a higher price.
NOT SO!!!
If you don't reply, I think:
1. You're not interested in selling, which is always a lie, because if I gave you your fantasy number, you'd be selling, right.
2. You don't understand how the game really works and not what they teach you in domain selling 101...therefore, I will offer you LESS and just wait. If I want the name badly, I'm calling you on the phone anyway.
I have found, as a general rule, that adult sellers are usually a lethal combination of arrogance and unsophisticated, especially the better the name. Granted, they're bombarded with offers, but even later on in the process, they've decided that simply owning a good adult name means you as a buyer should be on your knees. Generics owners are much easier to deal with, generally.
People completely underestimate the importance of timely replies and professionalism. Don't want to sound like a sycophant, but safesys of this very forum was the model of professionalism on a recent sale. Initially turned me down, simply saying I was offering too little; then weeks later pointed me to an auction. All the way through he was very obliging...
The simple rule for me is: If you don't oblige me, you make less money off me in the long run, because you're a jerk. Relationship selling is the best way to make money. Not the only way, but the best way.