Kam_123 I have had good luck purchasing new Pixel devices from Google's online store, and also good luck purchasing used Pixels from eBay, but only devices that are listed as "factory unlocked", from high-volume sellers, with high ratings and a no-hassle return policy.
My reasoning is that there are professionals out there, professionals know how different kinds of "unlocked" are different, professionals test devices before selling them, and professionals would prefer to fix the occasional problem to preserve their positive scores.
There are other reliable sources. Quite a few people here report good luck buying new devices from Best Buy, for example (though not all countries have Best Buy).