Mistakes I've made

Not preparing crystals or diamonds before a major event that has challenges that require gym or elite gym training to complete!
Not buying scholarships when their value is down from quests such as from the scout challenges!
Tapping into a retired supply too much such as fire cossie without considering that later on it may run out!
Not hoarding certain retired items like enchanted deca plushies! Relevant to the previous sentence.
Not realizing that after an event, more people leave and thus everything becomes harder to buy and sell.
Trading for any unpopular species without asking others' opinions.
Not considering overoffer or price matching on wanted ads when people get desperate. Use mm preferably to avoid this! I still love wanted ads though.
People get greedy, so they will definitely ask for more than what you're putting up!
Just before an au upgrade ends, people will get desperate to get le makers! Chibs maker rose by 2 mil! It's crazy. Definitely need to hoard those and not just rely on selling each for 12 mil for minimal profit
People always say they are a crappy restocker, just to fool you into thinking they are slower than they really are.