These are the II rules referred to earlier.
As mentioned, II can and do check for Exchange and Getaways being rented out and they do cancel reservations and suspend membership when they find these (so yes, sometimes people do rent them out and II don’t pick up on it, but it is against their rules and a big risk for both parties)
“Guest Certificates may only be obtained for personal or noncommercial purposes. Failure to secure a Guest Certificate where required for a guest of the Member when the Member does not plan to occupy the Host Accommodations (including instances where the Member has been issued Confirmations for multiple units having the same travel dates at the same Member Resort), or obtaining Guest Certificates that are used for commercial purposes may result in termination of the membership and cancellation of any existing Confirmations, including those with future travel dates. Guests of Members who arrive at a Host Resort without a Guest Certificate will be denied access to the accommodations until the Member has purchased a Guest Certificate from II.”