I would think any damages would be charged to the card the renter puts on file. Failing that then the owner would be the next in line, but since you and the broker both have the renter's contact information it would not be difficult to pursue legal action against them if necessary.
I agree with CarolMN, I doubt any broker would hesitate to help in a situation such as that. It will vary from broker to broker exactly how they would handle such a situation and whether they already have anything in place to prevent it, that would be a good question to ask the firm you are considering working with.