Share a workbook and update real-time

So I currently have a workbook that I would like to be able to share with multiple store locations and allow each one to access it at the same time. From there I would like them to be able to see the updates in real-time almost like google docs. But I also have a lot of macros controlling this workbook. One of these macros adds new sales to a sales history sheet. I don't want people overwriting the sales that have already been completed and recorded. Any suggestions on how to do this?
I have never used an online database. Would that be the route I would have to go? and if so, where could I start?

I haven't ever been able to get sharepoint to work for these kinds of purposes at my work. I have never used access either. Is there a way to convert my spreadsheet to an Access file while keeping all of my macros and all the same functionality?

So I think I found a workaround for my problem. There are only 3 or 4 locations and the sales aren't happening every minute so a conflict is much less likely to happen. And only 2 of the locations will actually make sales while the other two are storage.
So each location will get their own copy of the spreadsheet. In the spreadsheet I am making a macro that exports the data when a sale is made to a spreadsheet saved on DropBox. It will then pull the data from the dropbox file and update the location's spreadsheet with all other location's sales included. I am just in the beginning phases of this but it seems to be working for my purposes. I will try to let you know how well it works within the next week or so. However, I think this will really only work with a few users.

Yeah. I just plan on having it add the sale to the last line of data on the archive spreadsheet and then copy all that is there into the location's spreadsheet. I think that this way I will be able to avoid any duplicates. I will include an update button as well so they can update their records if they want to see the most accurate inventory levels.