When I worked as an auction house valuer stamps were the bane of our existence.
Really difficult and time consuming to value and treasures could be easily missed. Ordinary, run-of-the-mill, stock books would either not sell, go for peanuts, or, in the rare case when we'd missed something..... but they had to be spotted. An auction house with a track record for good results in philately is the best bet just as with coins.
I have my father's and my brother-in-law's collections sitting in my study waiting for me to make up my mind what to do next. Either submit them to auction and hope for the best, leave them where they are or just hope that a grandchild may be interested.