You've probably been subjected to many hyped-up claims about how easy it is to make money on internet. Simply grab a product, put a photo of it on a web page, and thousands of eager customers will flock to your site, credit card in hand!
Obviously it's not that easy. The internet is a wonderful tool -- but just like in 'real world', some types of products sell better than others. Consider following:
__Is there a more convenient alternative?
Products that aren't easily available at a local store have a better chance of selling online. Not only is it often more convenient to simply drop in and pick up whatever you need, you aren't hit with additional shipping or handling charges (which can be significant when shipping heavy products).
Pet food is an example: my household of pets dictates that I'm often running out door to pick up more food. At one point I investigated ordering it online -- imagine, door-to -door delivery! -- but in end, I decided against it: not only would I have to wait for it to be shipped to me, but shipping costs were nearly as much as food itself!
__Is your product something a customer needs to feel or see before making a purchase?
A simple example is a wedding dress: most women want to try it on, walk around in it, and simply see how they look and feel in dress before they make a purchase.
That's not to say that you won't be able to sell something that customers generally prefer to see or feel; if your marketing is good enough, you might actually make a great many sales. However, customers that discover that your product doesn't fit or didn't meet their expectations are likely to drive up return rate -- which means less profit and more hassle for you.