The short answer is: You can't.

Lies and scams are not moderated (see quote from the rules thread below). You only have to go in to Reputation for 10 seconds to see thread upon thread filled with lies and trolling about several different members. Your only option here is to create a self-moderated thread, but selling from a self-moderated thread is generally viewed as being somewhat shady since you can police the comments and delete anything negative that might be said against you. Your best option is to do what you have done, and simply directly respond to the allegations.