What do you think of a real estate agent who tricks one into signing a sales authority?

I recently had an agent & his rep list my home for sale ...during negotiations I told them I could only pay for advertising 'on sale ' they did not reject this...(they're rip off merchants ...what do they do for their commission?) Anyway the rep wrote 'pay on demand' beside 'due & payable'...I pointed this out twice saying that is not what I agreed to and he said ...(and I quote) "Don't worry about it!' I thought that meant I would be billed 'on sale' as we had spoken about moments before and signed. Three months later I began receiving accounts ...many letters flowed back and forth and I finished up saying I would pay it off after he gave it to a collection agency. I have reported them to Consumer Affairs in two states. If all responsibility rests with the signatory ...then one can ert that no responsibility rest with the proponent ! Surely this isn't acceptable...the law is stacked against the people ...what can be done about such s?