[drc]

There are a million things that can be said about the subject, but I will pick three.

To prevent losses, avoid investing in what is hot now, what's the in thing now-- especially if it's modern.

Buy at good prices. No matter what it is is, it's not a good investment if you buy at 3x going price. At 3x overpaying, you'd still lose money if it doubled in market value in a week.

Sell items once in a while, so you get a feel for what things really sell for. People who never sell often have their heads in the clouds about value and usually overvalue their items, while people who actually sell items have a real world view of what they'll get and make wiser purchases.

A positive of selling at an unexpected loss (which has happened to everyone, by the way!) is it can make you wiser for the future.

My old saying is the worst thing that can happen to a newbie stock investor (or casino better) is that his first pick wins money, and the best is it loses money. If his makes money he's suddenly sure he knows it all and is a stock picking genius. If it loses money, he is now aware that he can easy lose money, doesn't know everything and has to be careful and smart when making future picks.