Rule(s) about buying BIM software?

For example Revit (or AutoCAD products) is available through several resellers, but to me the resellers job here is not just offering the product (hopefully at a good price!) but assisting product usage, so if you take this view, the reseller is very important as each I think will offer you different levels and kinds of service. Good luck!

ArchiCAD sells only through its Graphisoft offices around the world so there's not actually much choice there. Revit has many resellers all over the world.