Buying a car radio on secondhand

on the 10th April 2015 I bought a second hand car radio at a cost of R750.00 and I was assisted by their employee, and I asked the Guy does he know how to install the Radio as I didn't know the policy that I was suppose to install my Radio to the qualified technician. the guy did install the radio as I was not happy with the way it and its operational then I went back to the same shop I explain my situation and the reason why I don't want to keep the radio , I explain to them that I want to buy the new radio on their shelf I was saying as I have paid for the one I don't want at least I will pay the difference for the new one I have chosen, so the assistance helped me when we get at the counter the cashier refused to take what I say he said I must explain to the manager and we explain to him, but he said I should sell that radio to their buying department, I went there and was surprise to see that they are buying that radio for R300 and I paid the new radio at a full price. Cash Crusaders is not helping us just reaping people.