I tried to unload some from 40 years ago-—practically worthless. If you have pristine blocks from pre-1950, you might be okay.
I have sent out packages with ten or fifteen stamps on them. My mailman just laughs.
I figure that because I am causing scarcity, someone else’s will go up later.
My wife is a huge stamp collector, and she was told by a guy who owns a stamp shop in the Twin Cities that any stamp made after about 1949 is not worth more than face value.