How can you prevent being ripped off when you are touring in a city?

Don't act like a newbie/inexperienced tourist. If the locals offer to sell you anything, just look and say nice but walk away. Don't buy anything from anyone that seems sketchy. If it's too good to be true then it is. Even if you're lost, don't act "lost" that's when bad people can take advantage of you.

find a trustworthy friend, collect related information before you make a trip, check the market situation, be able to tell from real or fake, avoid buying things in the evening or dark.

Consult with your tourish guide before you shop

Be informed. Google is your friend.