What design pattern and design principles to use?

I have to create an ecommerce kind of application in which first user will be registering so a new Contact for that will be created and he will be selecting different products available which might have validation whether that contact can take product on the basis of some conditions whatever it can be anything and the validation are going to be a lot as below.

The list of product is huge and almost every product has different kind of validations.

Can some one suggest me what kind of design pattern i can leverage to make my code extensible or redeable ?