Is it safe to contact a seller outside of Amazon?

No.

Sometimes when buying you'll notice sellers having an additional note asking to be contacted outside of Amazon. Do not contact them, as they will try to execute many different scam attacks, usually involving asking for you to pay outside of Amazon, or using Amazon Gift Cards.

Is it safe to contact a seller outside of Amazon?
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Never try to order Amazon products outside of Amazon, and especially never try to pay for orders outside of Amazon. Even if the seller asks for it, or promises a good deal, it's a known scam. Ignore the email or phone number provided.

P.S. On Feb 2, 2017 Amazon announced that it no longer accepts condition notes for all new products, effective March 8, 2017. This is a reaction to the increasing amount of sellers using them to scam buyers. Marketplace Pulse reported that this change contributed to the decrease in successful scam sellers.