You print luna and 'burn' terra. Or when people get terra you print terra and burn luna. The luna tokens supposedly have value because they get a share of the transaction fees.

The printing of luna when people sell terra should be offset by luna having been burned when the terra was printed. Ideally (when the price of luna is growing) then you print less luna at sale time than you burned earlier when the terra was printed. This decreases supply of luna, making luna worth more.