An enduser is a person or business who buys a domain to use it, not to resell it. You can find them in a number of ways, but probably the easiest is to type in the keywords in google or yahoo and find out who is buying ads on those keywords.
Often you hope that they find you . You can do this by noting on the domain that it's for sale, with contact information, or on your whois information, or by posting links in your signature here on NamePros as you will see many people do. You can also list your domains for sale at sites like snapnames or sedo and endusers can find you that way.
If you have a decent domain, they will find you. End-users can be persons / corporates who are willing to use your domain name for their own presence / corporation, so often the offer of end-user is much higher than it is for a domain reseller.
Although its been explained. The Term End-User is derived from the domains last owner. If your in this business, your in it primarily to resell a domain name. Ideally, you want to sell it to the last person who will ever use it - the End-User.