United Kingdom

http://pages.ebay.co.uk/help/sell/listing-variations.html

United States

http://pages.ebay.com/help/sell/listing-variations.html

Australia

http://pages.ebay.com.au/help/sell/listing-variations.html

After listing onto eBay, the listing cannot be changed from a single listing to a multi-variation listing. It should be ended before adding variation.

Adding up variation options to multi-variation listing is allowed after the listing has got sale record. Be aware, the sum of quantities from all variations cannot exceed 100. Otherwise, you may fail to revise listing.

The following eBay categories may apply multi-variations 

http://pages.ebay.com/sellerinformation/sellingresources/multiplevariationslist.html