>So far, online stores I have set up were having the database on the server. I mean, the main database was on the server. So, basically, it was maintained thru the Web by the administrative interface.
>
>Now, I have a situation where the client as its POS system in the store, and that is the main database. So, basically, we have to duplicate that across the wire to the server. So, I will have a module installed locally which will synchronize the Web server every 15 minutes. This is good for up to a certain number of items. If the client has 10,000 items to sale, then, we would have to think about an alternative as synchronizing that across the wire, with the related images, would take a lot of time.
>
>So, the question is, when we have that much items, would it better to consider moving the main database to the Web server instead and having the POS application in the store reading the data from it?

If you mean, the sole database would be on the Web server, with no local copy in a store, that probably isn't acceptable. The purpose of a POS is to reliably and quickly process sales transactions. Internet connections go down too often. Also, latency could be a significant problem.

>If this is not possible, what are the alternatives?

Some databases support replication e.g. SQL Server: http://msdn.microsoft.com/en-us/library/ms151198.aspx