(Why we don't have advertising)
Cookies are used by legitimate sites to store small bits of data on your PC that can be used to "remember" things. Like who you are, whether you're logged on, what's in your shopping basket, etc.
Cookies can only be accessed by their owner; Ebay can not read Amazon cookies, and Amazon can not read Ebay cookies.
However! if advertiser.com has an ad on Ebay and an ad on Amazon, it can access its cookie on both sites.
Not only that, it can also read what's on the page at Ebay and Amazon. This is how you can be on Amazon and see an ad for shoes like the ones you were looking at on the Argos site. Advertisers do this in order to "enhance your shopping experience". At least that's what they say, and I suppose it depends on how cynical you are as to how much you believe them.
If Amazon has an advertisement on it's login page, that advertiser can actually listen in on your log in, and get your credentials.
Companies like Amazon and Ebay don't have advertising on their log in pages, and now you know why.
Since almost all of our pages show personal information, we don't have advertising on 3rdKey.com