Selling of a used item?

I sold my treadmill last night - I used the treadmill the night before. I showed the 2 men that came to pick it up it worked great. They payed me then loaded it in the back of a truck in a down pour of rain didnt tarp it or strap it down got it home and they said it wont work and want there money back... I feel like they way they transported it is why it doesnt work.... just want others opions?? thanks