Refusing to return products

I recently bought a pair of running shoes that are too big. After running in the shoes for the first time (1 km) I noticed that it was I was getting blisters. When I wanted to return them I was told I could not because they had been used. Prior to buying the shoes, I was not offered the running assessment. I was just told to fit on the shoes and jog for 100m in the shop. This obviously can not determine whether the shoes are suitable. I was also not advised on shoe sizing by the sales person. I have now spent 2000 Rand on shoes that I can not run in because of a poor sales consultation