Two weeks ago, I received a postcard from Amazon—or, at least, I thought it was from Amazon. On second glance, I realized it was actually from one of the marketplace sellers who operate on Amazon. They offered me a tempting deal: they’d send me €20 ($21.65 USD) via PayPal for a review on an oil sprayer that only costs €10 ($10.82 USD). Here’s how they explained how it would work: I write a 5-star review (no length requirement).I email them my order number, a screenshot of my published review, and my PayPal address for payment.Sounds like an easy win-win, right? So, I tried it. It only took Ama…