"They don’t give a single damn about sellers — especially resellers" - Resellers by definition are a business, so as a private seller this shouldn't impact you to the same degree as business sellers on here.  After all you are not seeking a "profit" but to recoup a few quid from your second hand items.

Rather than coming to the boards with an ill-thought-out diatribe your time could have been better spent researching and presenting your items better.  Your reliance on AI for the descriptions does you no favours with inaccurate and overblown descriptions and 20th century sales waffle rather than helpful measurements.

For example: your "Very Rare Elizabeth II 2000 50p Public Libraries", a coin with a £4,586.72 opening bid price.  Very rare? - 11,263,000 were struck for circulation.  The value of the chipped and marked specimen you have for sale is exactly ..... 50p.  Even an original uncirculated example in the it original presentation pack purchased for £4.95 is now worth £7 at best.  Many people realise the true value of such an item and it shows you in bad light as a 'chancer'; which will not help your image as a seller.

If you are using promotions on low value items, especially in the second-hand clothes category the money on promotions would be better spent on reducing your price to be more competitive.  Promotions are a tool best used by businesses who have multiple items in 'stock'.

Your lack of sales and visibility will be mainly due to the negative feedback you received.  It is not wise to have the same item for sale on two platforms at the same time; and is in fact against eBay's policies - you cannot blame eBay's metrics for this.  This resulted in an upset buyer who certainly let his feelings be known - the good news is that this should drop off your metrics soon, which should then restore some visibility.

Good luck in whatever you do and wherever you go.