We have sold a few poker and gambling related domain names through one of our other sites Pokerdomainnames. You can see a list of what some sites can reach there but it's basically down to what someone is prepared to pay. A couple of simple things to know though. The .com is the only domain extension of any real value although you may wish to protect yourself by registering other extensions if your site has a strong brand and others may try and trade on that. Spelling something the wrong way doesn't work. Keep the domain simple so that it is easily rememberable and easy to say and understand. Generic domains are the most expensive at resale.
Agree with most of that, though obviously highly generic .co.uk names have a lot of value too. e.g. I know of a certain UK poker domain that has recieved offers of 7 figures. I also know of other gambling/lotto related names that have recieved similar offers... All with .co.uk extensions.
Bear in mind that if you are registering the name for main use in the UK market, then the .co.uk extension can actually be worth more than the .com counterpart. Still, if you can, always buy both versions of the name.
Finally, be very careful in buying a name that might already be someone elses trademark. e.g. Taito Corp of Japan recently sent me a cease and desist letter for spaceinvaders.com - meaning I had to give it away for free. What a stress and waste of money!
I'm currently in the process of trying to re-acquire a domain name for Mad Marty. It's an incredibly frustrating when the owner of the name doesn't reply to repeated emails. Especially when the name is of no use to you, and is someone elses Trademark.
...Edited to remove some domain details...
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