It's completely legal and very useful to give out your VIN to people who would like to run a quick check on the history of your vehicle. A VIN number is 17 characters long and contains information about the manufacturer, model year, type of engine in the vehicle, etc. Some people are concerned that giving their VIN number or having it looked up is unsafe or bad for privacy, but this isn't true. While some websites will have ads in them, there won't be any sites that just offer free VIN lookup. The website you're using will tell you if they will put ads on the site before you enter your VIN, so don't worry about it. However, you should know that you are giving out information that identifies you to the world. VINs are more difficult to change, so be careful about who you share them with.