It is a bad strategy selling a merchandise at $3 but charge $24 postage. Many Hong Kong sellers like to do it when starting, many of them have come and gone within a year.
People will know sooner or later that you overchage on postage. They will not come back any more, let alone that they have the rights to report you on eBay.
To make a decent and long-standing business on eBay, you need to build a large base of return customers. To other long living powersellers I know who sell on eBay for years, return customers contribute over 50% of their monthly sales.
Answers to your specific questions:
1. 是否一定要list很多online auction才可容易賣得出product呢?
Yes when your account is new. You can shift your listing from auction to store when you feel a significant portion, say 10-30%, of business come from return customers.
2. 收費廣告呢?
Those features are meant for high value merchandises, such as camera, luxury watch, jewellry, car or house..... Your $3-$30 a piece garment surely can't afford the fee.
3. 你們通常會list多少product呢?
More the better.
4. 每個月在ebay拍賣費用幾多呢?
Spare about 10-12% of your total sales as expenses on eBay and Paypal. Later you will know the expenses are more than just the listing fee, final value, and commissions
圖片參考:
http://hk.yimg.com/i/icon/16/3.gif
Also spare about 5-10% a month as your damage provision, over lost shipments and fraud payments.
5. 每個月純利有幾多呢?
If you are taking a price-driven strategy like what you are doing now, I guess you can't earn more than 20% net over a very small monthly sales.
As far as I see many competent Asian sellers are making 50% net profit, +/- 20%; much depend on your type of inventory, control of fixed overheads, and efficiency of your logistic process.
Good luck
圖片參考:
http://hk.yimg.com/i/icon/16/26.gif