Is permission needed when using collaborative artwork for commercial sales?

Yes. Other artists who have contributed to the artwork must give their expressed, written permission. Please have the contributors contact prints-permission@deviantart.com confirming permissions to authorize the print. Artists' permissions can be sent to prints-permission@deviantart.com

Please ask the artists you have collaborated with to send an email to prints-permission@deviantart.com with the following info:

Subject: Permission for "your devname"

Body: The body of the email should include their deviant name, and the image which they give their permission for you. One of the biggest problems we get is people sending emails but only including their real name. Needless to say, this makes it a tad confusing. We highly recommend using our sample permissions document, which you can download HERE.

It is recommended that you ask the artists to CC a copy of the permission email to your address so you know that it has been sent and you can verify it was properly sent to prints-permission@deviantart.com. We also recommend you and the other artists work out print matters before submitting the print to prevent any approval delays.

Please also see the following FAQs:
FAQ #257: What sort of permission do I need to use someone else's work?
FAQ #193: How do I prove that I had permission to use someone's work?