eBay are running a six week promotion where you’ll only pay the cost for listing a single item regardless of the quantity you list.
Items have to be identical, but you can list any quantity on fixed price (BIN or SIF) listings and only pay the listing fee for the first item. The promotion runs from the 12th August (today) until the 23rd September. There are a few restrictions so check the Ts & Cs for your particular categories.
Don’t forget that on eBay UK there’s also free gallery until the 30th September, effectively this promotion enables you to list your entire quantity of stock for the cost of listing a single item.