Usually that message means that the seller is on holiday, but it could also be due to recent uncertainty about how Brexit was going to affect sales to the UK. A lot of small businesses (on eBay or not) were advised to suspend sales into the UK until the situation was clarified so as to avoid being landed with the costs of customs clearance and returns of rejected consignments if the buyer didn't want to pay duties.
Now that it's clear that there will not be duties, things should settle back down (although deliveries will take a little longer)