1. Perhaps think twice before requiring (or requesting) your potential buyers to do anything beyond making a purchase.
Because with every extra click you can lose buyers, streamline the purchase process.
2. For items that are temporarily out-of-stock, you can still likely return 200.
Unfortunately there’s no such thing as a “temporarily out-of-stock” HTTP status code. So what should you do with temporarily out-of-stock items? Like most SEO issues, the answer depends on your individual situation with consideration for the user experience