4.Design model 

The various design patterns that can be used in this are

4.1) Abstract factory : This is used for generating seller and buyer agents at runtime 

4.2) Strategy : This can be used to implement various winner determination algorithms depending on the various auction models that are chosen. Also this can be done at runtime 

4.3) Composite : This can be used to implement the creation of item sets or bundles in cases where an bid consists of more than one items

4.4) Iterator : this can be used to answer user search queries