I'm not going to give some money back to a guy after I sold him a broken lawn mower
Why wouldn't you refund money to someone who is unhappy with a product you sold them?
It's a lesson to learn from, I'll never rip off people again.
It's understandable to want a deal to be over with, but its wrong to take advantage of someone for financial gain. They may have been purchasing a used lawn mower because they couldn't afford a new one. Now you might have deprived them of being able to afford one at all.
I don't think he's going to buy anything from me anymore.
It is indeed a life lesson now, so it's great you are aware of that now. Even though he may not want to buy anything from you anymore, you can prevent future customers from getting ripped off now.