Increasing items that are for sale...?

Ok my parents run a second hand furniture shop in east devon we have two outlets; a warehouse and large shop in a small town (i wont state the name because it would be advertising) we raise money for a selection of charities by selling donated goods and giving a percentage of our profits away. My father priced an item at �70 and a woman came down and said that'ss a bit high do it any cheaper and my father said sorry no can do, she left and then my dad noticed it was ercol which ipensive, he researched on the internet that the item we had was going for roughly 200 pounds or more and so he increased the price to 129 pounds, the lady came back that day and said i will take it for 70 and my father said no sorry i've just noticed that it's ercol so i've changed the price you should of took it earlier when it was 70 pounds. the next day when i was working she came down with her husband and said you father said i could have it for 70 so i've come to pay i knew this was wrong and she made a huge fuss shouting about how it was disgraceful that we had increased the price she then left and later came back without her husband she started saying how it was illegal for us to increase the price after we had offered it to her for cheaper. now i heard that it wasn't if a shop had mis priced an item or mistakenly out of ignorance priced it too cheaply we can increase the price for the shop is selling them, we spend money running these shops and we pay employees to work for us and then the fuel to collect items and deliver, we offer free collection of donated goods so therefore i believe that we should be able to increase the price because it increases our profits which in the end increases the amount we donate to charity. My question is the customer was stating how it is illegal to price an item then increase the amount i don't believe this is incorrect and i would like to know if i am right or she is but i have no knowledge in the area please could some one with information about trading standards or is aware of the topic tell me what the law states. i would wish not to be given website links unless they lead straight to the relevant page. thank you