What Was Your Absolute Biggest Mistake As A Beginner?

Couldn't have put it better myself

JCSF , thanks for asking. After a twenty year lull in my collecting I got back in and found a local dealer that I could go to and gain knowledge. 1988. He started bringing in the new issues of Ireland and Germany. I still visited the US Post Offices for the US new issues. By 2018 the internet had become my go to source for my specialty collections. It has been a truly great adventure.

It 100% sounds like one - Thank you for sharing!

Not starting to create and collect maxicards ASAP...

Ask not what your mistakes were, instead ask what was the BEST lesson you learned .... for me it was to forget the flash of new issues, first covers and instead pick a country, buy a Scott's specialized tot that country or group of countries and hit that country from start to 1950 and buy the best quality you can afford.

Biggest mistake was not selling all my duplicates and unwanted material when ebay became popular in 2001 to 2004 . Should of sold a lot more at those top peak years ,just held back too much .

As previously mentioned, focusing on quantity over quality.

On a related note, my biggest regret (other than not starting to collect U.S. revenues earlier, before the huge run-up in catalogue values) is not focusing on the key items first, instead choosing to get "moar stamps". Key items are keys for a reason. They hold their value and/or appreciate. Get them when you can, as you may not be able to afford them (or find them) later in life.

Tied to use an iron to remove a crease. The ink melted.

When I was younger and trying to fill my album, I regret buying a big shoebox of US 3c and 4c plate blocks at a "specially discounted price" of 30% of CV, that the dealer said was the best collection of its kind that he had ever seen. Well obviously it was all common stamps, there was tons of duplication, and 30% of CV is still close to double face value for low denomination postage, so it turned out to be a very expensive purchase relative to its actual value.

Though as echoed above, my bigger regrets were not buying unique specialized items that come up for sale infrequently. Some that I have seen only once in the last 10 years and may never see again.

A Minnesota state fair cover display, with covers acquired by me over a period of 25 years. I stored it in a damp basement with the covers getting foxing on them... The covers became a small US stamp on paper mixture. Two things, don't leave covers in a frame and get a dehumidifier.

Crystal mounts, bad.
Taping ends of Crystal mounts, bad,bad,bad.

ditto to electroprecancels