Re: From a customer's PoV
"the shop has no idea who I am."
Wrong. Unless you wear a mask, they can see what you look like. Over time, they will recognise your face and might be able to gauge buying habits. The may not know *WHO* you really are (street, d.o.b., medical history etc) but they will know that face buy X, Y, Z and then leaves.
Same with on-line. Your "tag" (be that IP, cookie, customer number, whatever) is related to certain sales. This is a bit like your face in a traditional shop.
And the next moan is usually "tracking". Well imagine you are down the pub and bump into a the above shopkeeper. Do you really expect them to *NOT* remember who you are?
Whereas a shopkeeper might keep that information in their head, a company will keep it in a computer and that *DOES* have to be managed (i.e. kept secure). Which leads to....
"there's no need to retain that information, indeed doing so contravenes the DPA."
Also wrong. Clearly have not read the DPA (nor, I daresay, the terms you agreed to when you signed-up to use a site).
"So no, traders, you are not entitled to know who I am"
Wrong once again. Depending on what you are buying, payment and delivery method being used; they may well be entitled to know who you are. The law may even compel them to know who you are! Try buying methanol to find out why.
"If I look elsewhere and there's no alternative you'll get my spam bin address"
You appear to know that an email address isn't who you are, yet you object to giving someone an email address. You are a strange fellow.
"if I notice you sending me any spam there you'll get a brusque mail back, aimed as high up your org"
Well don't agree to receive it then. Or try clicking the little "Unsubscribe" button at the bottom. The thing that has to be there.
"My identity is not yours to manage. It's mine."
Your identity is yours (and the government's). Your records and interactions with another entity are shared. If you don't want to build up any kind of "identity trail", I suggest you find a remote mountain and speak to no one.