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In Argentina I was once charged the "gringo" rate for a bus and was later told I could have negotiated the price because locals pay less.
From an ex-colleague who spent 6 months in China I heard you need to negotiate everything, especially at markets, for cab rides and accommodation. I also heard the same about Mexican and Peruvian markets.
What are some tips to negotiating prices (for things or services) across cultural boundaries ? If it's for sale, is it safe to assume it's negotiable ? How do I know what the local value of something is, that is, am I being ripped off ?