I have personally seen- and paid- around 80% of new, but those items were in excellent condition.
Bear in mind if your items are available from FestoolRecon their prices are always 80% new, but they also include a warranty of one year. If you hate the government as I do, not paying tax on a private purchase may tip the balance, as depending on the value of the items tax may be considerable.
So for high value and in demand items 80% is a good starting point and festool themselves seem to recognize that, but if it is less desirable or in less common use you may expect to negotiate lower. I assume festool must have a hard time selling their Carvexes at 80% as they are always on sale, for instance... in comparison you will only see dominos and midi extractors infrequently and for very short periods on their secondhand site.