First situation: Domain of the backlink which you bought/sold expired
Problem 1: You buy an advertisement or a backlink from website “A”. How do you find out if “A” website has expired or the domain has been parked?
Problem 2: You sell a backlink or advertise a banner from your “B” site, but you will forget to renew the domain (which will also affect your client, because he will lose some link juice).