I think these situations are a bit different. When buying WOW on ebay you know what you get, if off corse transaction goes according to plan and you are not scammed (your money is taken without receiving anything). In case of cloud mining you buy the service thinking you will make profit when in reality you will probably get some coins that are worth less than initial investments. The reason there are so many referral links out there is because they get percentages when you register. So I guess its profitable if you bring several several people with you and get percentages.