I sold my S4 through Tootle and got a really good price, about £4k better than VW offered me for trade in (and £2-3k better than WBAC). My asking price was met and exceeded on the same day.
If you want to avoid having the dealer haggle with you at collection, you just have to ensure you are upfront and honest in your description. Simple really. I listed one alloy was marked (the rest flawless), a large chip in the windscreen and a few minor stone chips, providing photos of the alloy and chip.
When the (independent) dealer came to collect, he did a thorough inspection but commented to me that he was just going through the motions as it was obvious that it was exactly as I described it.
Obviously if you don't declare things like scratches or dents then you can't be surprised when the dealer haggles you down a bit...