Please Paul be a superhero today.... and do not risk parting with your hard earned cash, any sites that have to canvass touting for trade in this way, are not to be trusted. !
Only advertise it through well known trusted and reputable sources.
Tell them you're only willing to try them if they secure a sale of your car first with no fee, then bill you later once the car has been sold. !!!
If they are that confident about keeping their words of guarantee & finding you a buyer, they should jump at your offer......Shouldn't they. ???