Since you are trying to increase sales on a short period of time, you want to reach out to users who've interacted recently with your client's content and initiated a checkout process.

In this case, you want to launch campaigns for two specific audiences:

1. People who've registered but have not finalized the checkout process. This audience is probably your "low-hanging" fruit. Someone who has registered, but has not completed payment, probably just needs a small push to complete the purchase. 

2. Users who have read specific content related to client's products, are probably the second best audiences you should reach. These are users who have shown interest in your client's products and might be interested in making a purchase. 

You don't want to run the campaign on new audiences yet. So running a campaign to a saved or similar audience is not the right choice for your first campaign.