It's hard to give advice since so much depends on the buyer.
The first question you have to ask is how the price makes you feel? If you are making good profit at that price, then you should be able to sell it and not have too many regrets. I think too many people reject buyers waiting for the big money that may or many not come. If you think the price is low, and you can wait, then it may make sense to do so.
But most buyers are not going to tell you their highest price to start with, and since they have contacted you, they want the domain. The question is how much do they want the domain...??? And, how much would most people say the domain is worth?
If you think it's worth more, then you should let them know, BUT you should do so in a professional manner. If you say you want 5,000 euros or even 1,000 euros, then that may make them angry or give up and go away since there is so much difference between that and what they offered. But if your domain is really worth that much, then it's fine to let them know that.
See what the other TLDs are doing. Do they have sites? Are they offered for sale? What are the prices they are listed at? You can often use these to help you. If the .net is listed for 200 euros, then it will help you to raise your price. If none of the other TLDs can be registered, it also helps your price.
Let's say you think your domain is really worth about 250 euros. What I would do is respond to the buyer with as much positive things as you can say about the domain: it's a .com, it's short, it has so many OVT searches per month, it has links, it has PR, it gets x visitors a month, the domain is x years old, and it earns $x a month.
Say all the positive things you can, and then let them know that you really want to sell it, but you think it's worth say, 400-500 euros. And tell them to let you know what they think so you can set a price that's reasonable.
Then you need to see what they say, and start negotiating with them. Don't be insulted if they offer you 70 euros. Just lower your price by the same amount to 380 euros, or you can got lower if your target price is lower. Just be firm, friendly, professional, and not greedy and you should get a good price for your domains.
So, to summarize:
- Learn as much as you can about the market for your domain.
- List and explain every positive point about your domain.
- Set a price that will make you happy, then set a higher asking price.
- Don't be greedy, but be firm, friendly, and professional.
- NEVER be insulted by an offer or by a price. It will allow you to either counter effectively or to walk away from a deal, which we all have to do sometimes.
- Don't regret the sale! If you set a price that can make you happy, then don't ruin that feeling by thinking what might have been if you asked for more. If you have learned something, then the next time you can remember that for the new deal and ask for more.