Hang on - they want £80 off you and then give you some back *if you sell*?
Surely there is therefore no incentive whatsoever for them to sell your car? They only make money by *not* selling your car!
I've been hassled by people like this in the past and I've not yet heard of *one person* who has successfully sold a car through them.
With this type of company, I'd guess the only reason they want money up front is that they don't have a good enough business model to get money out of actually doing anything useful for you.
But hey, maybe I'm just a cynic and they're actually a lovely bunch of people.