You can go back and flip your scans around so they are upright also by editing your previous post by clicking on the edit icon above the post, the one that looks like a pen and paper.
The backs of the stamps have to be OK also. If they are thinned, have a rip, tear, crease, scrape, paper missing, reapired, all devalue the stamp, some considerably.
Do not think it is worth $100 just because it looks like the other stamp. Nope. There are subtle differences that sometimes you need a magnifying glass or loupe to see or to measure the difference in the perfs (perforations) or the paper type or the colour of the stamp.
If the paper has been attacked by mold (rust, toning, mould) that is a major devaluing and most would not want that stamp in their collection as the molds will spread and ruin other stamps over the years.
Now, knowing all that stuff I talked about above is hard to do and usually takes years before you know it all and have experienced it. You might never experience some things. You may be as happy a two pigs in a puddle just collecting a few stamps here and there that catch your fancy or that seem good investments (be careful of that word with stamps) or to just own them so you can look at them once in a while.
As for selling, look on
ebay for example, and search for a specific country's stamps and see what is selling and how they talk about them and whether they sell or not (look at the completed listings). Or ho to a stamp show or stamp club meeting Stuff is sometimes selling for 10% of catalogue.
The mint (post office fresh) stamps I see on your pages are not worth as much as mint stamps with no hinges attached to attach them to pages. Mint Never Hinged or MNH on
ebay. Otherwise they are the MH or MLH Mint Lightly Hinged or even MHH Heavily hinged.
Read in te front of a catalogue for terms and grading of stamps.