And yes, I know you can sell more than that in a single busy day - but remember that on a busy day you have to put aside the pricing and shelving work until things get quiet. So these numbers will change a bit. You need to do the data entry on every book, and in order to do that, you will need to be able to describe the book correctly. Your store will no longer be self serve. You will have to find the book for your customer in every case.
Communication takes a bit of time - either by email or by phone.
You will also have to make up an invoice, pack the book for shipping , and go to the post office. That's 19.2 books per day. Using the rule of thumb of 1 sale per day, per 1000 books listed - you'd need 19,000 books listed at any given time to maintain this level of sales. One of the unexpected barriers you'll run into, is that at some point you'll reach a maximum inventory number, beyond which you cannot seem to go. My own personal barrier seems to be around 5000.