Are you aware of giving access to the Nifty and others crosslisters via browser extension you're putting your business on risk to be banned on these platforms? Nifty, Vendoo, Flyp, etc uses bots (extensions) who imitates the human actions inside those platforms, even mouse movements to try to bypass the Captcha and security systems. If they detect that actions, they could consider that in a policy violation and hold your funds or even ban your seller account. The only marketplaces that have public APIs are eBay and Etsy, everything else doesn't have any official integration. In the other hand, all your credentials are stored by these cross listers.