I've spent a fair number of time in computer and photography shops, and also been in the retail side for the latter. Here're my tips for retail stuff!
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Always know what you're looking to get. Do your own research first. If you need to rely on the sales person to make a recommendation, you've already lost. There's always a community of enthusiasts to rely on. Find them.
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Always do a price check with various reputable sources first. That way you have a rough idea of the price range.
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If add-ons of any kind are being recommended, say no and research first. Don't fall to pressure tactics to get them. If the salesperson is geniunely being helpful with the add-ons, they'll be happy to let you come back another day to get it. If they're saying you have to act now, it's a high-pressure tactic to sell higher-margin products to pad their bottom line.
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If a similar but alternative product is being hawked at you, again, do research first then come back. There's always a motive behind that. Sometimes it's just a need to clear stock of an otherwise decent product, but usually it's because the profit margin is higher, there're sales targets to hit etc.