Glad you've picked up on that topic. We're closely monitoring the sale of antiquities on ebay and other sites like http://www.historyforsale.co.uk.
(well mostly it's just me!)
There is a huge export trade of UK artefacts (and indeed other countries) to the USA, without the proper paperwork; i.e. an export license.
I have to stress that the metal detectorists, are only plagued by a small minority. There are thousands who are actively contributing to the Portable Antiquities Scheme database, which records items found by these hobbyists and amateur archaeologists. Rather than damn them outright, proper liaison with these people leads to the discovery of really important sites and the advancement of their knowledge.
For example the announcement of the Cumwhitton burials -