Re: Buy "almost new", sell older
It's your money, and I don't care what you spend it on (I think iPads are fine, but they just don't meet my needs). However, you should know that that line of reasoning is unsound...
2012: buy an iPad for £399
2015: sell same for £160
That's £239 spent on owning the device for three years. However, those 2015 Pounds are not as valuable as the 2012 Pounds you bought it with: like for like, you really only got back £147 of your £399, not £160 (calculator here for UK inflation: http://www.thisismoney.co.uk/money/bills/article-1633409/Historic-inflation-calculator-value-money-changed-1900.html ). Your cost is therefore £252.
If your other option at the time would have cost you £299, but those are now worth only £80, it is actually that one that is "cheaper to own". (a cost over three years of only £220, or £212.50 if inflation is counted).
The "high retained value" myth is repeated over and over again, especially in big-ticket items like new car sales, but there is no such thing as a free lunch: you are paying more, all the way through the lifetime of the product. It just looks like you're not because inflation and lost opportunity aren't obvious.
Please don't take any of this as a criticism of your choice of device. As I said, I think iPads are okay. It's just that they're a bit too expensive for what they offer (to me), and an apparently higher retained value does not make them "cheaper in the long run" - expensive is always expensive.
Basically - the only valid reasons to buy something are because you like it or you need it, and accept that you paid more for an irrational reason (your own pleasure in owning or using the product). No mass-produced good is ever an "investment".