Try something like this
=IIf(IsNothing(Parameters!MinimumSaleDate.Value), "", "Products Sold Since " & IIf(IsNothing(Parameters!MinimumSaleDate.Value), "", Parameters!MinimumSaleDate.Value.ToString))
I know this looks daft but it evaluates all of the code regardless - So even though your code should work it's trying to set nothing to string
So If you use an IIf even though it will never be used it should work
Hope that makes sense
Cheers
Andy