If vibration causes nuts to constantly work loose, first add a lock washer, if it doesn't have one. Or, if there is room on the bolt, use two nuts, tightened toward each other so they lock solidly. You can also try wrapping monofilament fishing line under the nut before tightening it on the bolt. Another possibility is to try wrapping the threads first with the teflon tape that is used in plumbing.
Every year, the National Association of Attorneys General lists home-improvement scams as one of the top-ten consumer problems. Here are a few signs to look out for: Out-of-town or unknown contractors or builders; Contractors who use the term “special introductory offer,” “limited-time offer,” or who offer any discount to use your house as a “model home”; Contractors who want a contract signed quickly; Contractors who want to discuss the price of the job later; Anyone who uses high-pressure tactics or makes you feel uncomfortable or pressured; A company that demands that you make full payment before the project is finished.