Short of an insurance policy, how about a legal waiver/release of liability? You could require any prospective purchaser of your wheels to sign a waiver releasing you from any liability. You may also provide information on how to maintain a wheelset (truing, checking tension, inspecting for damage, etc.). Any such form should be vetted by an attorney, but it may mitigate your risk chance of being sued without having to carry insurance. I'm not a high volume bike flipper (I sell a half dozen or so refurbished bikes a year), but I've considered creating such a form for bikes I may sell on Craigslist.