Theory:

     The storekeeper slid off his stool. From the first instant he knew instinctively, unreasonably, that the man hated him; but out of habit he rubbed his hands briskly together, smiled and nodded.

     “Good morning,” he beamed. “What can I do for you?”
 
     The man’s shiny shoes squeaked forward. His suit was cheap, ill-fitting but obviously new. He had a shuttling glance and close-cropped hair. Ignoring Purcell for the moment, he rolled his gaze around the shadowy shop.

     “A nasty morning,” volunteered the shopkeeper. He clasped both hands across his melon-like stomach, and smiled importantly. “I see by the paper we’re in for a cold spell. Now what was it you wanted?”
Explanation:
 
When Mr Purcell saw the man inside his pet shop, he slipped off his stool. From the first instant, Mr Purcell knew instinctively and unreasonably that the man hated him. But out of habit, when he saw him, he rubbed his hand, smiled at the man, and nodded his head.

After the unpleasant feeling, Mr Purcell welcomed the man. Because that was his job, and he needed to please his client. So he offered him a pleasant morning and inquired, “What can I do for you?” While Mr Purcell was speaking, the man, without saying anything, stepped forward with his shiny shoes. It signifies that the shoes he wore were new.
 
On the other hand, the man wore a cheap suit that didn’t fit him well, but it was new. It meant that the coat was loose, and it didn't suit him well, despite the fact that it looked new. Also, his hair was closely cropped, and he had a shuttling look. Even though Mr Purcell remained in front of him, the man neglected him for a moment and looked at the shadowy pet shop.

Later, Mr Purcell came forward and voluntarily said, “A nasty morning”. Then he smiled thoughtfully at the man and clasped both hands across his melon-like tummy. Furthermore, Mr Purcell stated that the weather was cold and that had been reported in the newspaper. After telling about the weather, he asked the stranger, “Now what was it you wanted?” He inquired because the strange man had been silent for a long time and was staring only at the pets in his shop.
 
Meanings of the difficult words:
 
S.No
Words 
Meanings 
1
StoolA seat without a back or arms, typically resting on three or four legs or on a single pedestal
2
Instant Happening or coming immediately
3 
UnreasonablyIn a way that is not fair or acceptable
4
NodSignify or express greeting, assent, or understanding by nodding
5 
Ill-fitting Of a garment of the wrong size or shape for the person wearing it
6
Shuttling glance Constantly looking to and fro
7
Cold spellA period of cold weather
Reference:
National Council of Educational Research and Training (2007). Supplementary. I Want Something in a Cage - L E Greeve (36-42). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.