Why didn’t he tell them he didn’t want to host the equipment anymore?

There is always some risk involved with keeping your stuff at someone else’s house. They could break it, accidentally throw it away, or resent you for using them as an unpaid storage unit. Many adult children who have homes and basements of their own keep their childhood stuff in their parents’ basements, much to their parents’ dismay. It’s one thing if they live in a tiny studio apartment, but if they have an attic and they live nearby, there is no excuse for not moving their stuff out of their parents’ house. 

It’s also quite difficult to split something with people who can’t share it in the long term. If you buy a TV with your roommates, only one of you will take it when you all move out. The person who takes it can pay back their share to the others, but does that really happen in practice? If you’re going to buy something big and expensive, it’s usually best to buy it for yourself and let others borrow it, rather than splitting the cost so a couch ends up with three different owners. 

The teens in this story, including the younger brother of KnownCheesecake9947, thought the gym set they bought would be safe at their friend’s barn, but their friend’s dad decided to sell it without asking whether they wanted it. He wasn’t even willing to give the kids their money back, which is really rotten.