Well they don't ' allow' it, it's against the law to sell counterfeit goods anywhere in the UK, online or elsewhere.
eBay have 7 billion listings on at any one time, so impossible to check each and every listing aside from the fact they do not physically see or handle the items.
eBay act on cases opened by buyers. If a buyer opens a case when they have received a counterfeit item, they are guaranteed a full refund if claiming within 30 days. Should that seller keep receiving those cases, eBay take note, remove the items, and ban the seller permanently from their site.
eBay have a recent Authentication process some high brand items enter into, this checks the authenticity if it fails it is not sent onto the buyer and a full refund is made.