These multiples are the result of information, in other words reputation. This reputation is subject to measurements. We measure it all the time and put numbers to it. When you buy a painting, you should know these numbers. And for the approximately 2,000,000 painters who don't possess a number, you should know that as well.
Ignorance is no excuse.
Ask your lawyer or tax accountant about ignorance of the law. You will get a single answer. Ask an art dealer about the same thing. You will get several answers. Work with the one who agrees with the above professionals.