Selling out of stock goods

I bought furniture at their store in Heidelberg on 2nd February and paid cash. I am now told other pieces of furniture are out of stock. Why did they sell me something that's out of stock? Their after sales service is appalling. I only just saw reviews from other customers complaining about the same thing so it seems that's how they operate i am so angry and disappointed and no one can tell when my furniture will eventually be delivered .