display sumif items in messagebox

I have a problem that I hope someone can help with. I have a sheet that gives a total using a sumif and I want to popup a message box that contains the values of the cells that were called by the sumif.

I have the code that will list all the cells in the range but not just the ones called by the sumif.

I have attached a screenshot of the sheet and the VBA code is below.

I have looked at precedents but in my real spreadsheet the sumif criteria key is on another sheet.

In the picture attached A8 contains the value SUM=(A1:A7) but should contain =SUMIF(B1:B7,D1,A1:A7) and the msgbox should contain a list of the values picked up.

Private Function RangeToString(ByRef rngDisplay As Range, ByVal strSeparator As String) As String

    'The string to separate elements on the message box,
    'if the range size is more than one cell
    
    Dim strMessage      As String
    Dim astrMessage()   As String
    
    Dim avarRange()     As Variant
    Dim varElement      As Variant
    
    Dim i               As Long
    
    'If the range is only one cell, we will return that that
    If rngDisplay.Cells.Count = 1 Then

        strMessage = rngDisplay.Value
    
    'Else the range is multiple cells, so we need to concatenate their values
    Else
    
        'Assign range to a variant array
        avarRange = rngDisplay
        
        'Loop through each element to build a one-dimensional array of the range
        For Each varElement In avarRange
        
            ReDim Preserve astrMessage(i)
            astrMessage(i) = CStr(varElement)
            i = 1 + i
            
        Next varElement
        
        'Build the string to return
        strMessage = Join(astrMessage, strSeparator)
        
    End If
    
    RangeToString = strMessage
    
End Function

Private Sub Worksheet_SelectionChange(ByVal Target As Excel.Range)
    If Target.Address(True, True, xlA1) = "$A$8" Then
    MsgBox RangeToString(Sheet2.Range("A1:A7"), vbCrLf), , "Items"
    End If
End Sub