It depends on what kind of products you make. This is very important.

Different varieties of warehouse receipts have different validity periods. Some varieties can only be written off after delivery and turned into cash (can not be delivered and sold again), and some varieties need to be written off and re-registered with warehouse receipts, which requires a certain fee.

For example, storage fee, delivery fee, warehouse receipt registration (possible), margin cost occupied by 4-month position, value-added tax cost and so on.

If there is no problem with the validity of general warehouse receipts, they can be bought and sold in September, but we must correctly judge the market. You have to pay the storage fee for holding the warehouse receipt for 4 months, and the delivery warehouse can't keep it for you for nothing. As for the storage fee, you don't have to pay (variety) if possible (the goods are in the warehouse).

And from your arbitrage point of view, you should also consider the cost of value-added tax (low in May and high in September).

There is still a lot to pay attention to. Arbitrage is not something you want to do (especially cash). Generally speaking, closing a position (even at a loss) is better than making a delivery.

Generally speaking, it is safer for spot enterprises. Of course, after you fully understand it, the opportunity is still great ... I hope it will help you.