A friend has over 20 yrs of mint and silver proof sets. He wants to sell them

Probably from 1990 on. Just trying to get a quick education so I dont look stupid.
My question is.
What years are worth getting?

And I dont know what to pay for them since I have never bought the modern clad proof sets,
or the silver proof sets. or mint sets with the Unc coins.
I would like the price a dealer would pay for the sets because I would sell some.
Thanks for any help