Anyone can take a picture. But not everyone can sell them. For the most part, the pictures which sell are sharp, clean, high-quality pictures, of multi-megapixel (for possible blowups). And in the case of stock photography, they may be concept or travel pictures. Concept pictures are those which depict an idea or a story. Travel pictures are those which are for travel destinations or tourist attractions.
For an amateur, one way to sell pictures is to have post these in photography sites which also allow the sales of prints. This means that the picture is on their site, and when somebody takes a fancy to it, he can order a print of the picture and this is delivered to his address. There are variations to this, with the pictures printed on mugs, for instance.
This would require not only a good photo but also a subject which is worth printing. It may or may not fit the requirements for stock photography, for instance, but it should catch the attention. And should look good when blown up as a print.
The other alternative is for the photographer to sign up with a micro stock photo website. There are a lot of stock photo websites on the internet. It may be hard to choose which one to submit to. But choosing which website to submit a picture to should not be a problem because you can submit any photo to multiple micro stock photo websites.
Most of the micro stock websites do not have exclusivity clauses. And besides, when it comes to copyrights, the photographer is the owner. The problem there is that in order to earn from stock photography, there has to be lots of submissions. So if you’re thinking of earning through stock photography, you might want to re-think about submitting to more than a few of the top sites.