Use google maps & street view to familiarize yourself with the area before traveling there to gauge what kind of neighborhood it is. If it looks sketchy or appears to be a high crime area, avoid. Be cautious with or avoid isolated, remote areas, especially after dark.
Be alert and mindful of your surroundings.
Don't advertise that you're carrying cash by taking it out and counting it, or patting you pocket every 10 seconds.
Buying/selling with someone who participates in the hobby is likely going to be safer than a random individual with no history in the hobby.
Take a friend with you if you have concerns.