formula to take list of values (rows) and place, separated by commas in single cell?

There is no native formula to do that in Excel. I recommend a udf. Here's one from Harlan Grove

Function aconcat(a As Variant, Optional sep As String = "") As String
' Harlan Grove, Mar 2002
Dim y As Variant

If TypeOf a Is Range Then
For Each y In a.Cells
aconcat = aconcat & y.Value & sep
Next y
ElseIf IsArray(a) Then
For Each y In a
aconcat = aconcat & y & sep
Next y
Else
aconcat = aconcat & a & sep
End If

aconcat = Left(aconcat, Len(aconcat) - Len(sep))
End Function

So after putting the code into a module
=aconcat(A1:B2,",") would give the contents of A1,B1,A2,B2
Does that work for you?

Yes, this is VBA and UDF stands for User Defined Function.
ALT + F11 opens the VBA editor. Insert> Module and paste the code into the text window that opens up.
Then close the editor (no need to save)
Now in your workbook, give it a try
=aconcat(A1:B2,",") or whatever your range is. If you want a space after the comma
=aconcat(A1:B2," ,")
Let us know if you have problems.

Now you're using UDF's. You're officially an Excel Wizard.