I have no love for ebay, but here's how I see it, and I sell a lot.
If any of that stuff is important to you, you should always check terms before selling on any platform - ebay seller fees have totaled close to 14% for around ten years now - perhaps longer. In markets where I can avoid ebay seller fees I do so, but ebay allows me to reach many, many more customers than any discussion forum classifieds do, so I use them quite a bit. Also, the prices I get in discussion forum classifieds are overall so much lower than what I can get in ebay, that the fees are a wash (I have no idea about knife forums, but I'll find out at some point

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(And ebay doesn't charge sellers tax - they take that directly from the purchaser.)