As a consultant for the formation of private collections?

[Varvara]

Tell me, please, as a consultant for the formation of private collections?
They work professionally, ie know the subject thoroughly and make recommendations to the client themselves, or according to the wishes of the customer buying pleasant work for him?
They take a percentage of them, if yes - then what? Is it worth using them or you can go very inquire the price and to buy as far as possible?
In his questions, I mean the art of investment.