The vendor asks for your information before selling you the part. There's two different regulations when it comes to this kind of stuff. ITAR is for pretty much anything defense related. It's not easy buying something ITAR as a regular person as most companies would probably just refuse to sell it to you unless you worked for a company doing defense work. People have gotten caught in the past by buying things in the name of their employer (or a fake shell company) and then selling it to places like China. EAR is easier to get a hold of as these parts are considered commercial dual-use technology. For example, a new microprocessor has a temperature range that exceeds the typical milspec range (-55-125C) so it's great for high temperature environments like downhole drilling equipment where it can get really hot but this chip would also work well inside the engine of an F-35. The govt has a list of specifications that if your commercial product meets them, it's classified as EAR. EAR is much more lax than ITAR as the govt doesn't want to hamper the commercial sector simply because a technology is new and better but the sames rules apply as to where these products can be exported to.