I am very new to stamp collecting and did not realize that was considered a cancellation.
I say this without malice (hard to believe maybe), but you don't need to know anything about stamp collecting or philately to know that's a cancellation; you only need to know how to mail a letter.
Also, more generally, words like "valuable" are so relative that they don't really mean anything without context. There are collectors that may spend $50,000 for a single stamp or cover and not think twice, while others can't afford to spend more than $5. If you were close to the original collector and have a sentimental streak (Lord knows I do), that alone makes the collection valuable.