Summarize:

If it’s a major operator with a well-established site, just bow out gracefully. However, you might get lucky and find that the domain was purchased on a whim, as a backup, or without careful consideration, in which case you may be able to negotiate a deal. Before so much as hinting at a price, simply email to ask whether or not the domain is for sale. If you are known or can clearly be linked to a thriving business, create a generic alternate email address through which to contact them, as your success might be leveraged against you. Be aware, however, that an informal-sounding email address is more likely to be regarded as spam or junk mail. According to internet entrepreneur James Siminoff, there are four basic haggling scenarios:  The owner suggests an unreasonable amount. If this is the case, counter with what you think is fair instead of low-balling it. It’s no secret that domain names are valuable real estate, so your underwhelming offer is unlikely to make the owner take you seriously. The owner asks you to suggest a price. If this is the case, they want to sell and are likely to try to negotiate up. Suggest 20 to 30% below your bottom range and let the haggling begin. The owner asks for less than you’d prefer. Accept, but not too enthusiastically, or they might begin to suspect they’re being too generous. The owner suggests exactly the price you’d prefer. See above. Even if you casually agree to buy the domain via email, the communication might be used against you in court as a legally-binding contract should you change your mind. Until you’re absolutely certain that you want to make a deal, agree to buy the domain provided that all the terms are agreeable. This will leave you an escape hatch if things go south. If the owner out-and-out agrees to your price, the email becomes an enforceable contract.
Find out who owns the domain. Contact the owner. Negotiate a price. Be extremely cautious while communicating with the owner. Get the owner to agree as soon as possible.