Congratulations on getting a nice offer to buy a domain! It never hurts to
brush up on your negotiating skills.
If you answer too quickly, and counter with a small increase in price ( as you did ), then a seller could see you as desperate, inexperienced, pliable and willing to go lower then their asking price. A clue that you might be interpreted this way is long lulls between replies to communications spanning many days or weeks and exponentially talking down the value of your domain.
By educating myself and negotiating wisely I turned a few hundred dollar offer into a $20,000 sale over a 2 month period. Good luck with this and don't lose hope, but most of all don't let the negotiating process take its toll or you'll end up selling for less then your domain is worth.
