Global market items prices have a leading comma if they are 3, 6 or 9 numbers long (any multiple of 3 i think).

This is quite outside my wheelhouse, but i'm guessing you need a number formatting function call there.
....depending on whether the stored value is an integer or string.
Where value is your stored number, presumably you could also pass in the language to (so the value is delimited according the user's locale).

This is quite outside my wheelhouse, but i'm guessing you need a number formatting function call there.
Code:
value.ToString("n0", new CultureInfo("en-US"));
or
String.Format(new CultureInfo("en-US"), "n0", value);
....depending on whether the stored value is an integer or string.
Where value is your stored number, presumably you could also pass in the language to (so the value is delimited according the user's locale).