Wom 27 is exactly right here. The stamp collecting hobby is a long way from its heyday in the 1950s and 60s, and interest in the hobby has waned considerably in the last 20 years. The American Philatelic Society currently brags that they have 28,000 members, but neglect to say that they were double that size two decades ago. The average age of collectors is between 65-70.
I have a modest stamp collection that I pieced together between the late 1970s to 1990s, including a sizeable number of “First Day of Issue” covers. I’ve got dozens of different varieties of Palomar Observatory covers from 1948 that I collected. Anyway, they are all currently worth a pitiful fraction of what I paid to acquire each one. A quick look on eBay shows that most don’t sell, and the few that do generally bring in about a buck or two. Unfortunately, most stamp collections are nearly worthless unless you have a few rarities.
I buy quite a few items on eBay that come from sellers who are coin dealers and/or collectors, and I can tell many of them used to be stamp collectors - because the packages or envelopes containing my items would arrive plastered with old stamps used to pay the postage. Over the last 10 years or so, it’s gotten more and more common. There’s nothing sadder than to see stamps from the 30s, 40s, 50s and 60s cobbled together to make up the needed postage for the packages, clearly raided from a once cherished collection, because they’re no longer worth any more than their face value.