The trade-in price history of P8 Max generally shows a decline. In other words, the used prices of this device are dropping as newer models come to market. It is only very rarely that a second-hand electrical device will increase in price; this usually happens for example if it's a collector's item. Therefore if you are looking to sell your P8 Max then it probably makes sense to do it sooner rather than later - before the next price drop.
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