Bid or Buy \*****ing"from me."""

In September I wanted to sell my laptop, which I did however not through Bid or Buy, but one of their competitors. I received an email about a month later saying my ad has expired but due to my time, I was not able to respond to the email. Once I did respond I was threatened with legal action as they wanted there commission although I sold my laptop through someone else. I have been in contact with them over email but they are rude and uncooperative. If I don't pay this commission soon they will blacklist me and hand my case over of debt collection. I am extremely unhappy. They give you an option to fill out a SNC form (Sale not completed form), however this must be done in a certain time period, which I was not able to do due to time related issues. I am not willing to pay that commission as they state you only pay if our item is sold through them. They further state that a SNC form needs to be filled in, in order for your account to be credited. Since I did not do this I have to pay for a service that did not benefit me. I will never ever use them again