If it's like Windsor, the larger sale items such as broods, frames etc can be pre-ordered and will be boxed for collection. Those items will also be available on a pallet on the day if you want more or need to count the knots. Some additional items will be only on the day, not preorder.
Preorders mean less danger hauling heavy stuff if most is presold. The no preorder items mean there is still a buzz at the start and impulse buying. From their end, preorder means you are committed to turn up and gives a good idea of numbers. They may also be packing in more non-sale items than otherwise that they won't be subsidising carriage on. They have to put time and materials in picking and packing but less handling and shorter queues on the day. Seems to work well.