Paypal allows you to sell and redeem gift certificates
What about building a plug in to support this in marketpress, and memberships, prosites and other wpmudev plug ins?
Not a replacement, but an interface to paypals gift cert functions
There have been many requests for gift cert functions, and paypal has done the hard part... So building on that would be great.
And avoids the secure data issues of keeping gift cert values stored in wordpress.