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Hi pixellated,
If you sell your photos through the suggested library sites, then you must not use any watermarks, borders or special effects. These site will place their own watermark and will be removed when the image is sold to a client.

As mentioned to "eric-boucher", if you look at 500px and private websites it is up to you to decide:

  1. Do I want to protect my image and block / protect it with a watermark?
  2. Or do I want the viewer to enjoy it and spread the word...

What I suggest is finding a suitable solution for different occasions if you don't watermark - there are probably more trick to try, but just of the top of my head:
a. Make the image smaller - e.g. smaller x-by-y pixels, or lower quality
b. Put a smaller signature on the bottom - see what I sometimes do at Vividrange
c. Use more expensive "watermark" software to hide the signature (something I haven't done)

But more important is - what is your target audience. Market for them and make money.
Again, hope my little advice gets you going...

Very cool, thanks for all the tips!