Some of the online EBay vendors are selling genuine Amsteel Blue line, but it may be spliced from end cuts, and could contain two or three pieces (or more). So long as the splice is done correctly, then it's not a problem...if it's not then this can affect the overall strength of the rope.
*EDIT* I just looked at those pages you listed, and neither one of them are Amsteel Blue. They don't even list the manufacturer of the line. Their 1/2" line is still not as strong as my 3/8" line...I would spend the money and get a trusted rope from a trusted vendor. /*EDIT*
I just brought some new line in for my brand new Warn 9.5XP from
http://www.amsteelblue.com. The pricing is great, and I've bought some from there before for an old 8000lb chinese special winch I had. Bre (the girl I've been emailing with) has been great and I couldn't ask for better customer service! 85' of 3/8", and a 50' piece of 3/8" for an extension, as I lost some length when I stepped up to the 3/8" on the winch. Shipping was fast...coming into Canada through customs and all (it will be in my hands on monday).