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I know this is more of a lawyer question, however I figure someone here has probably done this and can give me a quick answer.

Lets take a moderately extreme case. I build an adapter for a website that translates the requests and markup into a more formal API. I then take this API I developed and build a mobile app on it.

So in short I take a site that doesn't support mobile. Build a mobile application, and then charge for the mobile application.

I am not talking about charging for any of the original product. Simply the extension to the product.

I ask because there is a site I would really like to build a mobile app for, because it would make my life easier, and it would be pretty gravy if I could earn some cash on the side from the work as well.

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When building on the product of somebody else, e.g. the example you gave of extending a website or even simpler cases like writing a plugin for Adobe Photoshop, there are several aspects you should be considering.

  • Legal: This is what Karlson already mentioned, make sure you are not violating any licensing agreements, the terms of service of the website you are extending etc.
  • Economic: By extending an existing product, your business lives and dies with that product. If people don't like to use that product anymore (for whatever reason), nobody will be using your extension, no matter how great it is and without you ever having done anything wrong. Also, the company makign the original product might decide to include your extension into their product; think about how many people would still be willing to pay for a Photoshop plugin, if the same functionality (or maybe even 80% of it) was already included right out of the box by Adobe themselves?
  • Technological: Unless you have some kind of cooperation agreement with the company whose product you are extending, you might find yourself playing catch-up whenever they release a new version of the product. At best you will need some time to chnage the way you interface with their product (making you customers unhappy by forcing them to live withou´t your product for a while), at worst, there is a breaking change that makes it impossible to extend the product the way you used to (making you tell your customers they can no longer use the thing they paid you good money for).

See also this Joel On Software article quoting this Dave Winer blog post Co-existing with platform vendors.

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I think you should put the following to the test:

  • Is your application violating any licensing agreements or patents?
  • Will your application be accessing protected content bypassing the security measures?

If the answer to both of these questions is no then I don't see there being any legal issues with your charging for your application.

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