How to import data from navision and then export to it to update stock and sales transaction

Hi 

I am totally new to navision. 

My customer has the following: Microsoft Dynamics NAV 2009 SP1 V W16.0 running on windows Server 2008 and using database  sql server  2008. They don't have a Developer License.

He also has a tailor made  VC# Pos application running separately.  He wants me to daily import the Itemlist ( itemcode, ItemDesc, ItemPrice, Stock) from the Sql Server to the POS application.

And at the end of the day he wants me to retrieve the sales transaction from the POS application as a text/xml file and then export it Navision for the update of its sales and item ledger including stock etc

What are the tables which we need to query to get the itemlist for the above request?

What are the methods to update data to Navision?

Please advise

Best Regards

Mahen