I get quite a bit of domain sales spam also, but I just deal with it as I do most spam: I report it. Generally after a while they stop spamming me. You can report it by hand or if you want an easy method you can use
www.spamcop.net.
When you report the spam, it will help if you make sure the domains they are trying to sell you are reported as well. If the domain is not registered, then this won't work, but often they have registered it and you can report them not only to their hosting company or domain parking service, but also the domain registrar. Publicdomainregistry.com and Internet.bs have been really great about suspending the domains of spammers, but some registrars may or may not suspend the domains.
One last tip to reduce spam and find out who is abusing your email address: Create a lot of email forwards that all go to your main email address and use different ones for your accounts and when you contact people. Make a note of what address was used when you get spam and you will not only know who to blame, but you can also delete the forward and cut off all the spam...!
