When a seller sells a presale item it means you can buy it now, but the seller won't ship it until they're done finishing production.
The reason sellers do this is to get funding for the production and generate hype, letting everyone know that they are working on a eagerly anticipated item.
You take the risk of ordering without knowing the actual item quality. It's also possible the item never gets finished. When shopping with an agent they should be able to issue a refund if the seller doesn't send the item at all.
The only reason you might want to do this, the only advantage, is to make sure you get an item if the quantity is limited.