It's another very popular question, but those who have the answer guard it with their life, as they should.
BTW, just for the record, I'm not one that has the answer!
This form of e-commerce is super competitive, in fact it is one of the most competitive businesses in the world, if not the absolute most!
The biggest mistake people make is that they don't consider it to be a business when they consider getting involved, it's usually seen as an easy and convenient way of making a few extra dollars.
If people actually new they type of sellers they are going up against I would doubt if 99% would even consider the option as viable.
Many members here will tell you that they have been researching online markets such as ebay for years, yes years. Some are now successful, some aren't.
One fact is for certain, you need to consider this a business and treat it as such. You need to give yourself a good grounding in business basics, because these types of skills will teach you how to research your market and identify sectors within it that you can target.
Business is rarely simple, it takes long hours of dedication, but can be extremely rewarding for those that persist. There are no short cuts and research is the king in this type business.
You will find members here who are quite happy to share researching tips, business basics and the like, and all will be of great assistance to you in getting started in this type of business, but you have to be prepared to buckle up for the long run, and really put in some serious effort if you have any hopes of being successful.
Once you've been doing this for twelve months or so, putting in all those hours of research and learning, then you will have an excellent idea why those that have done it before you will guard their data with their life.
This is a great forum to get guidance, plenty will go out of their way to assist in that, but no one will find, catch, kill, slaughter, marinate, cook, serve, cut and then eat the cow for you.