Trade-in value changes for the 40 most-owned phones, tablets, watches and laptops at Apple, Samsung, Google, Verizon, AT&T, T-Mobile, Best Buy, Amazon, Gazelle, Decluttr, BackMarket, Swappa and ecoATM, enhanced carrier trade-in promotions with the fine print and bill credit terms, value cliffs after new launches, condition grading and deduction policy changes, trade-in kit and shipping deadline rules, buyback price gaps between selling and trading, data wipe and account lock requirements, trade-in scam and lowball alerts and used device price trends on Swappa and eBay, with the device or program first and the dollar value in the title.