There are 2 issues here:
1. The company does not keep a list of their customers. It's a retail business, and customers are just customers, not clients in the corporate sense. The customer who walks into your shop to buy Gucci 3 times a year can walk to a neighbouring LV shop to buy LV.
2. Say, your shop deals with LV, but the same supplier of LV also supplies Gucci, but your shop does not carry Gucci. So this customer who used to patronize you for LV products now decides to buy a Gucci product from you (instead of going to Gucci shop). You somehow manage to get LV supplier to sell you the Gucci product and you are in fact bypassing Gucci, not your employer, who is LV.
Okay, we can cut that crap about moral duty. I think we all know it's immoral. The very fact that the employee does it under the table and not dare to let his employer know of such a dealing shows the employee is guilty conscious.
Whether the employee can be prosecuted under the Prevention of Corruption Act is my concern.
Short answer:
No, not corruption at all. Fire away.
Long answer:
Depends on your appointment and scope of responsibilities at your LV shop. If you're responsible as a manager for the bottom line of the shop, then your higher management will not like it if they know, for this is liken to doing side-line at the company's expense, which means time, opportunity, means, resources, which arises from your appointment at the company. So, in the final analysis, the company can and will fire you. But legally, it is not corruption, because it is not tipping into a company's transaction to carve out your own reward which you're not entitled to, because this is a side-line, and has nothing to do with the company. Hence, most appt letters usually pre-empt this by making sure the employee does not use company's time and resources to do his own thing.
On the issue of customer contacts, on the surface it would seem like the contacts belong to the company, and if the contacts are generated due to the very presence of the company and yourself, then since you're taking salary or compensation from the company, the contacts would belong to the company. If you leave the company, you should surrender all your contacts to the company, but that does not mean the customer cannot locate you in another company and establish a new relationship with you.
Certain companies had tried to write into appt letters to prevent ex-employees from 'stealing' customer contacts for a duration of time, like a year or two, but such restrictions had been made voided in Singapore courts before, as the court's rationale is that such restrictions prevents an ex-employee from been gainfully employed in the area of expertise and/or competence that he or she had acquired in the previous company he was employed with.