You sell a lot by offering a lot (and by being featured). Those websites take an outrageously large cut from the actual sale and you need to sell a lot to see a significant income. You need to offer a lot: it is pretty much like stock photography on a smaller scale yet the principle is the same. My guess would be through patience and a lot of high quality work (or social contacts, see above) This leads to a second point: How to get into that pool is the important question.

Those websites have a pool of talented photographers who's work gets featured and sold on a regular basis. If he decides to search for something really specific instead your work needs to better than the already great search results on page 1-3 and it requires him, the customer, to actually keep browsing until page 10 where your newly submitted work can be found. A customer can find really great work on the front page already. Also your work needs to found in the first place, again, curation is the key. It's a lot about perception, customers prefer buying from an artist who 'is someone'.

Featured artists sell their stuff because they have an established profile and recognition. The market is grossly oversaturated with really talented photographers and quality alone won't make you stand out.

You need to sleep with the curator: Why would I say that? Because unless you get featured on the front page regularly you have a snowflakes chance in hell to sell anything. You will never make any substantial money on those websites unless the following applies (the more the better): Until someone with actual experience weighs in ill stick to some educated guessing.

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