A million unique items? Wow. A well thought out schema and appropriate indexing is going to be key. That said, the amount of data isn't the issue, it's the amount of transactions. How many write operations (Inserts, Updates, Deletes) do you expect per hour or minute? That's the question you need to answer first. And how much data (in MB or G growth and change will you have over a defined period of time.
For a highly volatile OLTP system, that server isn't going to be able to handle it. Plus, I assume you will have other things besides the POS running on it (nightly jobs, lookups etc.), besides hitting it from the back end with your data imports.
Is this an upgrade to an existing system or a brand new app?
If it's an upgrade, measuring the current transaction volume and growth trends will help you determine your hardware needs. If it's a new application, you'll need to make some educated guesses. Scaling out can be done at a later date if necessary, but you'll need to have an idea well before it becomes necessary.
What database platform are you going to use? If MSSQL or Oracle, replication is pretty easy and dependable.