I queried it with the boffins this morning; they replied:
'MOD as a whole has a particular concern about the use of eBay. These concerns are that MOD employees are not allowed to trade and more significantly the, sometimes, inordinate amount of time spent by users on this site. Whilst the Acceptable Use Policy (JSP740) prohibits the offering for sale of items and services on commercial auction sites and warns that personal use must not interfere with official duties; DGInfo and the 1* DIRGE have directed that all eBay sites be blocked between the hours of 0630Z - 1800Z.
Anyone with a legitimate reason to access eBay during this time, is required to forward a Business Case to the DFN Gateways Team'
So, it seems it is more to do with excessive time spent perusing the site than actually flogging HM's kit on there. The night shift should be ok though........