Personally, pre-sales are probably not the way to go generally. You don't know when CGC will actually ship the books. You don't know if the shipment will be lost or damaged.
If you're going to do pre-sales, it'd probably be better to wait until you get the book IN HAND before accepting the money. (You can pre-sell them, but you tell the buyer to wait until the books come in to pay.)
Otherwise, you're kind of getting an interest free loan for whatever extra time (15-30 days?)
I understand needing to pay for the slabbing, but this way isn't it and the possibility of something happening is too great.