Business Data can exist in multiple Applications.  You can have a customer list in Excel and your eBay inventory in an Access db. 

You may want to keep your business data in these Applications because you and your employee's are used to using those tools. 

There is a programming language that is built into all Microsoft Office Application called Visual Basic for Applications.  It uses a technology called COM to help share data between applications programmatically.

This can be very advantageous and help save time and double entry of data.

Although there may be online tools to help you, you may want to keep the data on your local server.  A customized solution may be the right answer for your business built using the software that you already have.