Question

Is it advisable to use a .resx file to store common string references in a business layer class library? I typically only ever see these been used in the presentation layer.

Answer 1

One option to be to have a separate project in your solution, which has all the resx files for your whole solution. You can then add it as a reference to your business layer. In the Resources Project, you can write a wrapper around .net ResourceManager class, to return the resource value for your key. Something on the lines of:

public class ResourceService : IResourceService
{
    public ResourceService() {}

    public GetResourceValue(string resourceFileName, string resourceKey)
    {
         var resourceManager = new ResourceManager("Myresources", Assembly.Load("MyResourcesProjectName"));
         return resourceManager.GetString(resourceKey);
    }
} 

Then you can use it from you business layer as:

var resourceService = new ResourceService();
var resourceValue = resourceService.GetResourceValue("MyResources", "ResourceKeyName");

I did not have time to test the code, I have written on the fly, but it should give you the general idea. IMO, there is nothing wrong in having the resx files in a separate project.
Another way of approaching it, is to have .resx file in places where they are used. I find the separate project idea better, because that way, you can create separate folder for each type of language, and store locale specific .resx files in it.