Based on your description of the sales application you've built in Excel, you might be in a good position to build some experience with SQL Server. You can probably get your employer to buy you a copy of SQL Server 2012 Developer Edition (pretty cheap and a worthwhile investment even if they won't pay for it) and then you'd have access to all the bells and whistles to learn with. I'd justify it by saying that you'd like to be able to have backups and see if you can work with the data in new ways. Then the first thing you do is create a database, right click on it, go to Tasks > Import Data (I assume it's the same in 2012 as 2008 R2). Then pick your Excel spreadsheets and try loading some data into your new staging database.

Once it's there, try querying it using SQL. Do some reading on dimension tables and fact tables and see if you can build processes to turn some of this sales data into a data warehouse. From there, give Reporting Services and Analysis Services a whirl. There are tutorials on Microsoft's website that use the AdventureWorks database and you should get a good idea of how SQL Server does things. Wouldn't hurt to find some books too and if you're stuck, you already know where to go for help. If you plan to actually use it at work, then you'll have to get a license for a different edition and you'll also probably want to trash the work you've done while you were learning and start over.

It sounds like you already know how data works and how to do analysis. That's not as easy to learn as learning how to use a tool like SQL Server. I think that once you start working with it, you'll find you'll be able to not only do much more than you can in Excel, but it'll take less time and be more reliable. I'm sort of understating it, but I remember the moment when I first started to get a handle on how powerful SQL Server 2000 was. It was one of those "Ah-ha!" moments. If you go a route like this, then you'd have a project under your belt that you can talk about to a potential employer when you're looking to move into a role that's more BI focused.

Good luck