Sam Westcott here - head of development. What you're referring to does happen from time to time, but that's completely the fault of the email client.
What we do, in using a standard From and a custom ReplyTo, is the recommended best practice for sending email from applications such as ours.
If we used the owner's email address as the From, our deliverability rate would decrease, which means that your quotes would end up in guest's Spam/Bulk folders more frequently.
This is because a lot of major email providers (Yahoo, Hotmail, Gmail, AOL) check the "SPF Record" for the sender to see if they have authorized the sending server for their domain. You can read more about SPF here: http://www.openspf.org/Introduction
Most web apps now follow this practice. For instance, you probably get inquiries from VRBO. Notice on your inquiry emails that the From is always set to firstname.lastname@example.org and the ReplyTo is set to the guest's email address. When you hit the reply button, your email program is supposed to ignore the From and use the ReplyTo.