My dad was a solicitor. Not the type you see when you buy a house, he worked in house for oil companies, writting contracts for drilling in international waters. Or something.
I wanted to buy an album from Our Price (for those under 40 this was a chain of record stores). I was about 14. Album of the week was something like £4, and other albums were a bit more, something like £5.
I went to buy the album when it was Album of the Week, and they tried to charge me the higher price. I pointed out the poster in window that said the lower price. They said that only applied to vinyl (tfw window poster didn't say this). I wanted a cassette because I didn't have a record player. I went home because I didn't have any more money.
Told my dad this tale. The next day he met me from school (😭) and made me take him to the shop, where upon he launched into all sorts of legalese and quoting contract law, and invitations to tear and misleading advertising etc.
I got the album, on cassette for £4.
Our price added a footnote re: vinyl only to their Album of the Week promotional posters.
Anyway, what I was trying to say above, is that an advertised price is a start for negotikns. Pretty much always it's what people pay and move on. Notably, houses and cars are where people tend to negotiate (I don't know if there's different law for these) ; baked beans less so. A seller can refuse a buyer's offer, although a friend of mine did once succesfully negotiate the cost of a sofa (in M&S coinidentally). About 15 years ago MNers caused a run on the Mottercare website, when they listed some car seats with the decimal in the wrong place ie: £12.00 instead of £120. Mothercare didn't honour those orders.