The "interest" is the value added by being in a queue so that you can get $THING sooner than if you didn't reserve a spot.
Whether the value of the $THING (and having it earlier) actually justifies the opportunity cost of the deposit simply depends on the cost/thing/schedule and the buyer's wishes.
Whether the value of the $THING (and having it earlier) actually justifies the opportunity cost of the deposit simply depends on the cost/thing/schedule and the buyer's wishes.