Like the other member said if we knew the answer to this question we'd all rush out & buy a million units of this item.
Ebay is very competitive & it is becoming increasingly had to make money.
We have to compete against the Chinese sellers (who get subsidised postage). For example, lets say you wanted to sell phone cases. There are currently 15.8 million availble on eBay. Some in China are as low as 10 cents buy it now with free postage. If I wanted to post one to my next door neighbour it would cost me $2.00 (2 stamps on a large envelope). There is a 30 cent fee to receive payment via PayPal plus other ebay fees. As you can see I could not compete with a Chinese seller in this field.
The key to it is to find a niche product or offer something tthat few other sellers have. Once you have found the product(s) it probably won't take others long to cotton on and offer the same. From there it is a race to the bottom as to who's prepared to sell it the cheapest (make the least amount of profit).
I wish you well and if we knew more about what you're thinking of doing you might get more specific help (eg new or second hand, type/category of potential item(s), full-time/part time, hobby/professional seller.
