I buy and use 3D models, and I'd never heard of Poly. Free3D, TurboSquid, and CGTrader are better known. Poly isn't even on the first page of Bing search results for "3D model".
It was mainly for sharing Google Tilt Brush models. With Tilt Brush being shut down this makes sense.
It went open source: https://github.com/googlevr/tilt-brush