Refuse to exchange goods

I bought black trousers at Centurion mall from Woolworths. Bought it on my account. Did not keep the slip and didn't cut off the labels as I did not wear it. Lost a lot a weight and took it today to exchange it for a smaller pair. They told me it is off the system and can't exchange it!! Absolutely pathetic! Where do I sell it now, on the internet? Will never ever buy from them again!!<br> Milady's took back Jeans that I bought 16 months ago - now that is service!