One thing not mentioned thus far........if you have full sheets, keep them intact as full sheets. The complete sheets have more value than partial sheets, known by some dealers as "scrap postage." But again, the full sheets are only marginally more valuable than "scrap." You may also want to check in Linn's Stamp News, a weekly stamp paper. You occasionally will find dealer buy ads for this sort of material. As others have already advised, don't expect to obtain much more than face value, if that.
I myself
inherited a bunch of this very same material when my dad passed away 20 years ago this month. I've used maybe 1/4 to 1/3 of it on letters over the years. Now debating whether it is worth any time/effort to try and get the material better organized for possible sale ("translation:" likely a lot of work for little return).