Adding data to database using asp.net and sql-server
I would personally encapsulate your instantiation of MusicStoreDBEntities in a using statement so that the database connection is properly closed and disposed of.

using (MusicStoreDBEntities obj = new MusicStoreDBEntitiesObj())
{
    //code to add record goes here...
}


I think you should also pass in Instrument Item as a single parameter to AddStringInstrumentItems() rather than each individual property of that object so...

public static void AddStringInstrumentItems(instrumentItem item)
{
    using (MusicStoreDBEntities obj = new MusicStoreDBEntitiesObj())
    {
        obj.instrumentItems.Add(item);
        obj.SaveChanges();
    }
}


And in your instrumentItem class you want a constructor to instantiate it.

public instrumentItem(int typeId, int brandId, string model, double price, String itemimage1, String itemimage2, string description, string necktype, string body,
string fretboard, string fret, string bridge, string neckpickup, string bridgepickup, string hardwarecolor)
{
    this.typeId = typeId;
    this.brandId = brandId;
    // so on 
    // so forth
}