There's a UK-based site called Preloved. I've only bought 2 things from it so far, but they were both like new and pretty rare. Reasonably priced, too. One of the things I got was an OG Steam Controller, which was selling for ludicrous prices elsewhere since it was discontinued, but the person I got it from sold it at face value. I think your success at secondhand buying is stronger when you find wee sites like that that most of the internet is unaware of. The best advice I have is to type your search query into your desired engine, and immediately go to page 2 or 3 of search results and start from there.