About SQL Server 2005 Express

I developed a small application to manage inventory, accounting, payroll and so, for up to 5 users, running on SQL DB. Now I have some people interested for the software, they want to use it in their small companies. If I sell them the solution, do they have to license SQL DB ? or can they use express edition for free ?

You can distribute (or they can download and install) SQL 2005 Express. It is FREE.

You may wish to check out this resource:

SQL Server 2005 Express Redistribution
http://www.microsoft.com/sql/editions/express/redistregister.mspx