Is the iTextSharp DLL free to use and redistribute with my web application project which i will be selling?
Version 5.0.0 and up is licensed under the AGPL (strong copyleft).
Version 4.1.6 and previous are still licensed under the MPL/LGPL (weak copyleft).
So for a commercial product you'll almost certainly want to stick with 4.1.6 or previous.
Here's an archived unofficial citation.