It could be that someone has set a maximum proxy bid on that auction, so ever time you bid the proxy kicks in and outbids you.
Either that or it's a popular lot.
Papa
Dave
Try ebay
I've found a couple of Sigma lenses, that are Canon compatible.
Have a look
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=2939113045&category=4687
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=2938820381&category=4687