Is there any evidence for a "decline in sellers"? 

I sell as a business across many platforms and I find that different types of buyers are loyal to different platforms. On Amazon, price is not the issue but more how fast you can get the goods to the customer - and people search on Amazon rather than browse. Vinted is a very singular market on clothing and I am sure that will have had an effect on some sections of eBay. But I find in general that it is business as usual on eBay. eBay is a platform where it is largely price competition - a race to the bottom - and there is a constant flow of sellers collapsing when they can no longer compete at the bottom end.

I have no evidence, but I would guess that many private sellers who have been trying to run businesses without registering as a business have now discovered that they need to convert or face the coming storm. Margins built on the lower eBay fees for a private seller are by definition not sustainable when you register as a business. So some will drop off the bottom, no doubt, but as they do so eBay cleans its act up and becomes a better place for businesses to sell. The process will take time but I think this is the correct strategy for everyone.