https://www.purplebricks.com/terms/service-agreement
Scrolling down to first purple window ,payment due on which ever comes first either the house selling,stopping using purple bricks or 10 months.There is a fourth point that you must use their conveyancing services as well.
Nope just in funds.I looked into purple bricks,my understanding is you have to pay the fees when the house is sold or after 10 months of listing whichever comes sooner.Now if they havent sold your house in 10 months charging you for it is a bit of a kick in the pants imo.