"eBay's search is a mess and I actually think this is a major reason for their declining sales in the UK."

For some people it may not be a single reason that causes them to stop using ebay, but a dissatisfaction with the site in general - all sorts of little things that aren’t enough on their own to cause a departure, but together add up to it being more trouble than it's worth.  I’m sure the broken search function is one of those factors, plus the dominance of unwanted sponsored listings, plus the deceitful listings which offer, say, a 20-piece tool set for £1.99 then you discover it’s actually only a screw for £1.99 and the tool set itself is £50, plus the multiple listings of the same item from the same seller, plus probably a dozen other little things.  And if you do order something, it probably won't arrive when you expect it because of the unrealistic timescales that ebay gives. 

It’s just all so tiresome.  Sadly there’s nothing about ebay these days which makes shopping a pleasure, and once it stops being a pleasure, you stop buying and you go elsewhere.  I wish it were otherwise, but again sadly, I see nothing to suggest that anything’s going to change any time soon, if at all.