How to avoid HttpRequestValidationException in ASP.NET MVC rendering the same view which caused the exception

I just want to know how to validate (or clean) user input in ASP.NET MVC so that an HttpRequestValidationException will not be thrown regardless of the values submitted. For example, with a text input, if the user inputs <BR/>, it will cause an exception and the Yellow Screen of Death will be shown. I don’t want that. I want to catch the exception and to make visible an user friendly error in the current view, preferably with the controls loaded with the same values submitted.

How to create Encrypted PayNow button “on the fly” for Third-party customers, using Paypal NVP API?

I need to create Encrypted “PayNow” paypal buttons on the fly for a website.
I read all the documentation I can find on the paypal website.
I understood that I need to use the BMCreateButton ButtonManager NVP API.
But I’ve been unable to find any information, nor any reasonably simple and documented sample code, about HOW I’m supposed to call these API.