They can't sack you based on sales unless it's in your contract. Full stop. Then, you'd probably just be paid commission rather than contracting in terms like that to your employment: it's better for them if they've got someone standing around not selling anything (but still tidying, looking busy, talking, smiling, etc), and not getting paid, than noone at all.
I work at DSE (normal Dick Smith store, not a powerhouse); we don't have individual sales targets, but there is a store-wide sales target and until that's met the managers push everyone (a little), and then it's clear sailing.
IMO, Depending on your job, often little difference between the 'retail' (ie being a store with no pressure) and 'sales' side of employment. In the end, at DSE, while it's considered 'sales' by the head honchos, in the end if a customer doesn't want to buy, what're you going to do? How can you get persuasive about a $5 cable or $10 batteries or a printer when they find a better alternative elsewhere. From there, assuming you can be "selling" (ie pressure and metting sales targets, rather than just informing and being a porter for goods between the back room and the customer's trolley), and they expect you to be "selling", if they don't TRAIN you adequately, you can't "sell" anything.
Forget the sales, do your job, go home. That's all they pay you for.
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BTW, where do you work?