Can I buy resources at the end of the game to fulfill scoring conditions for the Sufficiency tile?
No, you can only buy resources to build tiles. You can't buy them to fulfill scoring conditions or to activate cards such as launchers.
PARTLY right. You can only buy resources to pay costs, BUT it can be any cost (tile purchase or activation). Two excerpts from the rules, emphasis added:
The Exchange is a “market” that helps players pay building or action costs when they don’t have or don’t want to spend the needed resources. It also allows players to sell excess resources for points.
Buy from the Exchange:
To gain a resource “credit” to pay an immediate cost, stack the indicated number of your point tokens (1, 2, or 3, making change with the supply as necessary) onto the next open spot aligned with the resource you want. This can be done multiple times as long as there are open slots, but ONLY to pay costs of blueprint tiles or actions—you may NOT buy resources you do not immediately need.
So while you CAN buy resources to pay for actions (which makes Launchers that much better, and will often enable money arbitrage), you CANNOT buy resources to fulfill scoring goals (that's not a cost).
Good question though!