I think there are two things you need to consider…
1. You are LUCKY to be approached by a CASINO that offers to buy your domain, and you should try to backtrack and find out WHY they want to buy it.
Do they think that the resemblence in the name will capture typos and lead them to the site? (there are many other reasons).
2. Once you have an answer to question 1, you should sit down with yourself and figure out whether at the end of the day you will be able to capitalize from THEIR assumption…
Or would you be better off just selling it to them and make a few bucks…
BUT – if this site name is a trademark and eventually you will be accused of (i believe the term is) domain squatting
then you should get rid of this domain ASAP. i.e. – SELL SELL SELL.
Interesting – let us know what you decide (if you are willing to share the info with us…)

by the way – I am not a lawyer… just a guy who’s been arround the internet for a while… so – better to consult a lawyer about the squatting issue.