eBay requires third-party authorization before SellerCloud can interact with your account. This basically means that you letting eBay know that you are using SellerCloud to manage your account and you are granting permission for SellerCloud to access it. This is accomplished by fetching a token using SellerCloud's eBay setup wizard. Once the third-party authorization is setup, you can see it in your eBay account on Account->Site Preferences->Third-party authorizations.
It is a good idea to look over your existing third-pary authorizations to make sure that there is nothing that will conflict with SellerCloud. On a number of occasions, customers had listings suddenly end or be listed with no history in SellerCloud as a result of conflicting authorizations.