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While he could legally negotiate to not charge delivery, he cannot legally waive sales tax. Under NV law, when parties enter into a contract of this nature, if you signed the receipt agreeing to the deal, it was your responsibility to read that document before you signed it. If you did not do so until you got home after you signed the receipt, you have no recourse legally, because you cannot introduce oral statements from a sales person to contradict what is in the written agreement under the NV law called the Parol Evidence Rule.
If you have evidence he told you this before you agreed to the sale, you know he will deny he told you that, then you could file a complaint with the Attorney General's Consumer Protection Unit for unfair and deceptive business practices and you could sue in small claims court for the unfair practices. If you do not have any evidence other than your word, I am afraid that you would not be able to succeed in court on this matter I am sorry to say.
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