I have a business plan for selling apparel over the Internet, with unique features that distinguish this idea from other sites. Before patenting this idea in the WIPO, can I tell about particularities with the investor, or is that a mistake?
You must avoid public disclosure of your idea before filing a patent application. The usual method of doing this when talking to potential investors is a confidentiality agreement. Have a confidentiality agreement drafted and have both parties sign before disclosure.
The short answer is no, not without having them sign a non-disclosure agreement.
There has been written a great article on this topic here: https://ocpatentlawyer.com/protect-idea-before-pitching-investor-licensee-buyer/ by James Yang, Patent Attorney.
Of course, from my experience, it can be hard to make a potential investor sign any agreement before disclosing some details about your idea/invention. It goes back to trusting the potential investor and whether you believe that they could describe your idea in great detail to others. You can limit the possibility of them sharing this, by ensuring that your meeting is not recorded in any way and that no pictures are taken nor images shared with the investor that he/she may pass on to others or make available openly.
Basically, you have to ensure that no publicly available information describing your idea is available by any means - and thereby, you have to eliminate potential threats to this.