eBay's outage announcements and status board tend to be slow to react, and rarely reflect problems with specific features unless they impact an entire category such as "Billing" or "Shipping".
There has been an ongoing problem with that particular feature on ebay.co.uk for the past two weeks. I suspect that eBay is rolling out some changes to Advanced Search, or else a change elsewhere has caused something to break regarding those types of searches.
In the mean time, you can search for a seller's items using a link like this:
https://www.ebay.com/sch/xxx/m.html
Substitute the seller username in place of the xxx above.
