Is this plagiarism or competition?
pasbesoin: I had this happen, at the idea stage.  Not that I necessarily would have proceeded, anyway, but it caused me further hesitation.A year+ later, the "competitor" site has not taken off.There is concept and design.  Then there is also execution.  If you believe you can exceed in execution, this, in my mind, is a valid reason to proceed.  Just keep in mind that your target market is not yours alone, and that this may influence your chances of success.  If you are still willing to proceed, I don't see an "ethical" issue here, as long as you're not simply viewing the competitor and ripping everything off.If you are concerned about perception, you might (I'm not sure about this, both from a marketing perspective and from a legal perspective) simply explain where your product comes from and why you are doing it.  People will believe you or they won't, but being straight-forward and proactive in this might pre-emptively blunt some criticism.I wouldn't mention or would be cautious in describing your competitor's decline to cooperate; it might sound negative and/or reactionary on your part.  Focus on what is motivating you and what you hope to achieve.