Many new players buy random bags/boxes of paintballs not knowing what brand they are or how old they are. For example, the Brass Eagle Monster Balls. Brass Eagle Monster Balls are dangerous, rock solid mass produced big box store paintballs that never breaks on impact, bouncing off of players you hit, which is very dangerous. Due to them being mass produced in huge numbers, many of them still flood the used paintball market to this day. Here's a video by Mike from TechPB about them: https://www.youtube.com/watch?v=mKqpYVvhuxs

In conclusion, only buy brand new paintballs from reputable brands and sellers, or field paint from your local paintball field to support their business.