Your customers can verify their domain using your domain.
Whitelabeling DNS records
First make sure your own domain is verified using the SPF, DKIM, and Tracking records on the Domain Verification Guide - Your own customers can then add the following to their own DNS.
SPF
Name: @
Type: TXT
Value: v=spf1 a mx include:yourresellerdomain.com ~all
Tracking
Name: tracking
Type : CNAME
Value: tracking.yourresellerdomain.com
Do not change DKIM entry.