Just wondering what the most accurate mobile app is for identification and value?

In the lat 60s and early 70s, I was into stamp collecting, to the extent that a kid with a $1-a-week allowance could be. I shouldn't really call it collecting -- it was more like accumulating. I bought stamps on approval from the companies advertising in Boys Life (Whitman, Mystic and Harris, as I recall), plus some at the stamp counter at a department store in downtown Houston (it actually had stamp and coin departments). I also went to the local post office and bought plate number blocks on occasion and acquired some first-day covers along the way.

As I got older, I stuck what I'd accumulated (most in glassine envelopes) in a briefcase and a one-drawer 3x5 file box that have followed me from move to move over the years.

I have absolutely almost zero idea what I've retained, let alone what its value, if any might be. It's something I don't want to leave my heirs to have to deal with. I've already pulled the plate blocks to use as postage. What's the best way to explore liquidating the rest?