One, or both, of the following two questions:
"What product(s), or service(s), should I sell?"
"What product(s), or service(s), will make me the most money?"
is asked by just about every person opening a new online business. Not only is there no simple, good, answer to these questions but any person who knows what they believe to be a good answer is not likely to share it with someone else. Choosing what to sell is either going to be the easiest thing you do in your business or one of the hardest!
If you find a hot item to sell, and get it to the marketplace before anyone else, then you are either very skilled or very lucky. There are very few instant success stories around and most of those are marketing hype or just plain lies. There is just no magic formula for instant online selling success. I hate to tell you this, folks, but for most of us building a successful online business takes work, time, and perhaps a little money.
If you already have access to a product, or line of products, that you believe will do well, or if you currently produce something, or can offer your own expertise as a service, then choosing what to sell online may be very easy. Whether it will do well or not is a topic for other sections of this site. The key point is that you have a product or service with which to begin. That's good.
If you don't already have access to products, don't make anything, and don't have a skill you can market, then you have your work cut out for you. Jumping in with just another copy-cat business that adds to an already flooded market is probably not a good choice. Competing head-on with large distributors isn't good either. That is, unless you can bring something unique and special to that segment of the market. Can you deliver better merchadise? Can you offer better pricing? Is your service significantly better and/or your shipping time (and cost) greatly reduced? Or do you add a feature that no one else has? If there is one or more "yes" answers to those questions then maybe you can consider selling hot items that everyone else is selling.
Sometimes there is a sales area the has lots of sellers but there is a great deal of room for other sellers because of the variety of items available to sell into that market. There is a current researgence of interest in vintage clothing and vintage sewing patterns. There are lots of sellers with this kind of merchandise on selling sites like Main Street Mall Online or eBay, etc., or with their own websites. You might at first think this market was already saturated. But because these are mostly unique items, often one of a kind, and because the original supply of these items is mostly gone and those in the hands of consumers are mostly worn out, lost, or destroyed and, therefore, also gone, there is a continuing demand for good pieces.
If you can source these at estate sales, yard sales, thrift shops and other local venues, and then offer them nation (or world) wide through your online business, you may do very well in an already busy market. The key, then, would be exposure -- which is covered elsewhere on this site.
There are two ways you can build your online business at least as far as product selection is concerned. You can focus on a single product line or range, or you can expand you view to encompass a group of lesser related items that will let you bridge across fads, sales trends, and marketplace changes. This latter may take a little more work but it is a more sustainable approach and will more likely give you a steady stream of income over a longer period of time. It may be a slower way to riches, but you will have more control over your income as your business grows.
To get you started you can always get some experience selling your own excess items and then the things your friends and neighbors wold like to get rid of. You'll need to work out a commission and cost assignment process for the latter but it will give you access to a quick and easy supply of items. The money you make will help you purchase your initial "real" inventory and the experience will go along way to getting you ready for the big time. If you are nervous about selling at all, you may want to hire an eBay Trading Assistant to get you started selling a few of your own items. Carefully observe what they do, what they charge, and how the process works. It's a great way to learn.
If you find selling general, used, items like this works for you and you wish to continue then you can find a continual supply of items to sell at yard sales, fleamarkets, and thrift shops. Some online sellers earn their total income selling used merchandise online. Some sell anything they feel they can make a profit on. Some narrow their selection to things they like and/or know about. Don't pay too much for things and know what you can ask for them online.
You can also focus on things you know or things you worked with. For example, I studied drama and film in university. At the time I wanted to be a movie director. Little did I know the journey life would lay out before me. It certainly didn't lead to movie directing but I have always been a film and theater buff. So, I might choose to sell videos, DVDs, film and theater posters or memorabilia. It would certainly be things I am familiar with.
You could look back on your career and the various jobs you've had. Think about things, concepts or ways of doing certain tasks, that you developed and that could be useful to others. You could develop those into products or services you could market. Sell what you know. Even if you are selling the sames things others are selling you can put your own personal spin, drawn from your previous experience and your creativity, and offer something different.
Look around your own home. There are lots of products there that you use regularly, that make your life easier, that you just had to have. Find out who makes them. Contact the company and find out how you can get access to these items to sell them online. You'll be turned down by a lot of places but keep looking. You will find one or more that will be glad to deal with you. It takes effort. But your efforts can be rewarded.