Back when I worked in the jewelry business, we would spend some time trying to talk people out of repairs or custom orders which we knew weren't worth it, or they would not be happy about. Quoting a little on the high side convinced most folks, but there were always some obstinate ones. Requiring half up front changed many of their minds. We did the work for the few who insisted and ponied up the payment. We would write "non refundable" on the ticket in large letters, before they signed. Sure enough, they often wanted their money back and would argue for quite a while.

If the bikes not worth it, you should ask for substantial money up front. Putting up forces them to decide if they really want to go forward. Should get rid of the ones who say go ahead and then walk away, too. bk