Yes, ISPs can see which domains their customers are browsing, and when they are in Europe, they even have to do it.
The data retention directive of the European Union forces all European ISPs to log all the connections of their users for at least six months for law enforcement purposes. Most European countries have already ratified and executed this directive. A notable exception is Germany where public protest and a constitutional complaint prevented it from happening till now, but the newly elected government has already announced that they intend to go through with it in this legislation period.
But your fear that the ISPs might sell this information to your competitors is likely unfounded, because EU privacy laws prohibit them from accessing that data without a court order. That means as long as your users activity on your website doesn't conflict with the law, the ISPs should (should!) keep their users connection data confidential.
By the way: Next year the European parliament will be re-elected. There are parties which want to get rid of this directive and others who support it. It's your choice which one you want to vote for. (Full disclosure: I am a member of a party which wants to get rid of it).