Then you could sue me, and I'd countersue you for
violating the GFDL.
What part of "add no other conditions whatsoever to those of this
License" don't you understand?
If you're currently forcing datafeed recipients to agree not to
redistribute the data they receive, then you're in major breach of the
GFDL. Not just the relatively minor breaches that have been going on
for so long, but you've subverting the very essence of copyleft.
It depends what you mean by "reselling the datafeed". If we're talking
about making a new datafeed from your servers to your client and
sending the data you get from the Wikimedia datafeed, then there is
nothing anyone can do the stop you, but that's not really reselling
the datafeed, it's just reselling the data. If you mean somehow giving
your customers access directly to the Wikimedia datafeed, so that they
are using Wikimedia's bandwidth, then the GDFL doesn't cover that -
it's that which I would assume is forbidden by contract.