Even if something is marked up wrongly in a shop they do not HAVE to sell it to you at that price although quite a few supermarkets have a policy of doing this and then changing their sel's
I once bought some really nice speaker stands for half what they should have been but they were marked up wrong. When I got home one of the feet was missing, I went back to the shop and the guy had realised his **** up and said pay me the extra £60 and I will replace the foot....
I loved those speaker stands and you didn't really notice the small piece of wood holding the right one level