There are a number of messengers in danger of being shot with a question like this.
First, unused U.S. stamps from about the 1940's on sell for less than face value, generally speaking. There are exceptions, but not many. Having full sheets doesn't help...sheets are too much for most collectors, so they typically go straight into the discount postage market. A dealer might pay 60-70% of face for most sheets.
The stamp-related "collectibles" are almost impossible to sell these days. There is very little market for the gold-plated this or first-day that.
There are a number of "so you
inherited a stamp collection" websites out there, which have as their primary mission the task of explaining to non-collectors that stamps are great fun, and very educational, but rarely valuable.
There are exceptions, of course. But I couldn't count the times I've been in a dealer's shop when he had to explain to a family that a decedent's collection really can't be purchased by the dealer, for any price, because the dealer can't resell it. It undoubtedly provided hours (years, really) of enjoyment to the collector, but in many cases the assemblage can't be marketed easily.
Setting proper expectations early is critical. That lets someone be surprised on the odd chance that something interesting pops up.
Hope this helps. Probably not what you were hoping for on behalf of the client.