Consider this: If you make an offer for a domain and your offer is accepted without question, wouldn't you think you may have paid too much? You will actually be doing your buyer a FAVOR in most cases if you counter their first offer if you do it in a professional and/or friendly way. I say this because if they get a counter and can then make a deal, they may feel like they paid more than they wanted to, but less than they could have.
The way people think about this process is much different in many countries, so it is good to keep that in mind and always to be flexible, yet firm. This goes for buyers as well. Say you get an offer for $500 and you feel the buyer has money and the domain is good and perhaps worth $1,000. You want to sell this domain you paid $10 for and $500 is not a bad price. If you counter at $1,000 you risk the buyer leaving because that is twice what he offered. If you don't care about selling, this is fine. But you want to sell so you counter at a more reasonable $750. If the buyer thinke the domain is worth $1,000 they may accept or they may only offer a little more. You then have to keep working until you cam make a deal you are both happy with.
If you can get the buyer to like you, you may get a better price. But if the buyer can get you to like HIM, you may lower your price.

The best thing is for you to both like each other and find that price that works for both. This is the best kind of deal, not the one where you make much more money than you expect. Why?
Well, if you are only going to buy or sell one domain, and don't care about other people, then try to get the best price you can. After all, you are never going to see or hear about this loser again, right?
But if you want the other person to also feel good as well as yourself, there are some good business reasons to do that. For one thing, they may buy more domains from you in the future. Or they may tell others about what a good deal THEY GOT, and how nice you were to work with.
Now, if you get an offer and you like it and don't want to risk having it go away, there are two things you can do that are not accepting their offer without a counter:
1) You can counter with a slightly higher price. If they offer $500 you can counter with $600 or even $525. If they won't increase their offer, you can walk away or accept it. They don't know if you will walk or not, so chances are they will pay more. If not, no problem you just tell them, "Ok then I guess I can accept your offer". They will be happy and you can be happy without letting them know you are happy.
2) The other thing you can do is NOT name a counter price. You can respond with something like "Thanks for your offer, but we were hoping to get a little more than that. Can you increase your offer?". They may offer more or ask you how much more. If they don't offer more, then they may be close to their limit. In that case tell them a slightly higher price and see what they say.
Try to not lose a sale because you expect too much or misjudge your buyer. I have lost quite a few sales because of this. Buyers don't make offers too often (unless you have good domains...

) so if you can make a sale for more money than you paid, you should generally take it.