It's been a long time since I studied it, but my recollection of contract law is as Kathrin says; when a shop puts its wares out on display it is just an offer to sell; you then must agree the price and it is only when the seller has accepted payment from the buyer that the contract becomes binding between both parties. Until then, no-one is under any obligation. I don't think it's changed since then.
I think that the reverse is also true; if someone makes a PO/SO but then changes their minds, they are not forced to go through with the purchase. Correct me if I'm wrong; don't think I'll ever be lucky enough to be in that position!
I think that there is sometimes some misunderstanding in stores due to so many different languages being spoken and different global customs.
As an example of when it can work in our favour, many moons ago when some friends of ours got engaged, they chose a beautiful ring from a certain well known jewellers and paid the deposit as it needed to be adjusted to fit my friend's finger. A couple of days later, DF's fiancé got a call saying that they realised that the pricing had been wrong on the ring and instead of it being about £5k it should have been £15k! Much to-ing and fro-ing between jewellers and fiancé ensued, but as we'd all just been studying contract law, he stood his ground and had to point out that as the company had accepted part payment for the ring, that was their part of the contract and therefore the price was binding. Result!