Have you already signed up to a broker? Which one? What domain? Did you have an offer on the domain?
There are too many variables in your question, feel free to private message me. Some private brokers may use such underhand techniques to make a sale and pocket the profit. Use a reputable firm if you are worried but in most cases brokers rely on reputation to ensure continued business in this industry.
I am a Private Broker but not looking for business as I am over loaded at the moment, here is what i suggest:
Post your domain in a forum and ask for them to review the price. DNforum.com would be a good place to start, these are domainers and will give you "EndUser" prices as well as domainer prices. This will give you varied valuations but you now have a ball park figure to play with.
It is a brokers job to get the best possible price, after all they get commission on the sale. A good broker will ensure the price he has from the seller is the very lowest he can sell for. This simply ensures that he has no need to continually come back to you for feedback and allows the broker a free hand.
Just because a broker says it is worth $x,xxx does not mean you have to accept. If you gabe the broker a price of $1,000 then he would probably accept and sell for more, this ensures you are a happy customer.
By all means send me the details and i can certainly point you in the right direction.
Best of luck