It does say in their policies tat they aren't allowed
"eBay Policy:
Used clothing may be listed on eBay, so long as the clothing has been thoroughly cleaned according to the manufacturer's instructions and is stain free. Used underwear including, but not limited to, boxer shorts, panties, briefs, athletic supporters and cloth nappies is not permitted. Listings that contain inappropriate descriptions will be ended."
If you list them without selecting the "used" condition - just leave it blank - I don't think they spot them often! ;)