When an event's AggregateOffer (top level) has no common "buy url" then the event's webpage url should be used. This can happen if the offers don't have buy urls or if the offers are different buy urls (one per day for an event series) and there is no common buy url for the AggregateOffer.
When an event's AggregateOffer (top level) has no common "buy url" then the event's webpage url should be used. This can happen if the offers don't have buy urls or if the offers are different buy urls (one per day for an event series) and there is no common buy url for the AggregateOffer.