Hi Augustin, welcome to the forum,
There'll be a lot of factors in appraising your collection. Everything you've shown here online, are used, i.e., cancelled, stamps. With very few exceptions, used stamps are usually worth a fraction of unused stamps. If you do have unused stamps, being hinged [mounted with a small gummed, glassine tabs] to the page is another factor that would detract from the value of the stamp. Although, many collectors collect nothing but used stamps — but mainly due to their accessible pricing [some collectors collect certain postmarks on the stamps]. Even mint [unused], with no hinge, stamps can vary in value due slight design variations or plate flaws, the clarity of the printing impression, centering of the image [with even sized margins on all sides], the condition of the "original" gum on the reverse side or the freshness of the paper, i.e., oxidation and other unplesantness. There's a host of factors in determining the grade of a stamp. Some factors would be difficult to determine from a photograph, ex., someone earlier mentioned your "Black Penny" looks to be a photocopy. Personally, I wouldn't judge it from an online photo, although, it does look a bit greyish, but if it is real and unused [as it appears to be] that one stamp could be worth some heavy coinage. A photocopy would be worth nil.
I bring these points up because you mentioned your partner looking at selling prices online. The condition of what is being sold may be different than what is in the collection you've acquired, for better or worse. Just some things to keep in mind. Perhaps your local library has a set of Stanley Gibbons catalogs in the reference section. Aside from median pricing, you can find information regarding how to judge the grade [condition] of your stamps.
I'm sure you've heard this before, I'll still put it out there. Any pursuit of collecting should be for a love of what is being collected. It's evident that the person who built your collection did so out of an admiration for the stamps; working in a post office probably gave him the opportunity to obtain loads of stamps from discarded envelopes. Stamps as an investment is a tricky proposal and should be pursued with some knowledge of the stamp-trade. And remember, just because it's old doesn't mean it's valuable. High values are usually due to rarity and/or a superb specimen of a given issue.
Sounds like a wonderful collection. Have some fun with it . . . good luck with the valuation.
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