There was a bit on the 'Today' programme about a row between Asda and Bloomsbury about the price of the latest Harry Potter book. No doubt we'll soon be getting another "why have you bought all those copies of the book at suppliers' discount when you could have popped round to the supermarket and got them cheaper?" or "why didn't you buy them off Amazon and get them a bit cheaper" letters from a councillor or one of the council's accountants.
The answer to the Amazon question's quite tricky to explain if the person asking the question isn't much fussed about the impact of supply times on CPA-related performance indicators and the potential costs of part-order deliveries. Councillors and council accountants should be much fussed about both these things. (Notice the use of the conditional tense there!)
The supermarket one's easy: you're not going to see 95% of the titles we've got on our shelves in the supermarket, they're not high-volume sellers and so can't compete with tins of beans for shelf space. Of course, if we were only buying occasional bulk orders of best-selling titles then that's precisely what we should be doing (and off-setting the processing costs with the discount). But we're a public library and we don't, which is why we negotiate contracts with book suppliers.