What are the various phases of consumer merchantile model and also differentiate between prepurchase interaction & post purchase interaction.   

There are three phase of consumer mercantile model as listed below:

1.  prepurchase interaction
2.  purchase consummation
3.  postpurchase interaction
 
Prepurchase Interaction

The prepurchase interaction for consumers consists of 3 activities:
Product/service search and discovery,
Comparison shopping and product selection, and Negotiation of terms.

Purchase Consummation

This model lists 3 activities in the purchase consummation phase:

Placement of order,
Authorization of payment, and 
Receipt of product.