I agree with PPs about being asked to give emails and postcodes at the till. With the email, they usually tell you it's to send you an electronic receipt, so it's easier to find if you need to query or return a purchase, but this is so obviously a lie. I'm amazed that it's legal to take somebody's email under false pretences that it's for a one-time single message and then to add it to your regular marketing-spam list without getting their express permission.
Some retailers just don't seem to understand the (you would assume) obvious fact that stalking people will make them avoid you completely, or at the very least seriously damage people's perception of the quality of their brand - which is presumably one reason why they spend so much money advertising it in the first place (i.e. to strengthen, not damage it!).
It's the same with online purchases. If I buy from a company and they subsequently send me their newsletter or latest offers every month or so, I don't really mind that - and it can indeed be a helpful (to me) and therefore profitable (to them) reminder that I might want to look again at their website to see if there's anything I want.
However, when I buy one small thing from them (especially if they're often the kind of companies that sell things that no one person needs very frequently - if ever more than once) and they instantly add me to their daily-spam list, I ignore it for the first few days and then just hit 'unsubscribe' (not that I actively subscribed in the first place) - meaning that they've completely lost that potential sales contact forever through their own idiotic stalking.
How difficult is it to understand? If people get an email every six weeks from you, they very well may think "Ah, email notification - who's that from. Ooh, it's XXXXX company - I wonder if they have any good offers or interesting new products." If they get an email every day, they open it and think "Not them again - just leave me alone, will you." They then associate your brand name with hounding, desperation and causing annoyance and then promptly unsubscribe and/or block you.
Not to mention the e-tail sites that keep showing intensely irritating pop-ups to inform you that "A customer in Lisburn is also currently looking at this item" or that "Sixteen people have bought this item this week." On what planet do you think that I, a (potential) customer could care less about what your other customers do? It's valuable information to you; I, however, couldn't give a stuff.
I can just about abide it if stock is very limited and the message is that I might miss out if I don't decide to buy now (as with used goods being sold by individuals on eBay), but these are invariably shops that have vast stocks of popular new items as well as easy access to unlimited re-stocks from their suppliers.