If the CPU is working and all of his pins (legs) are intact then a 2nd hand one is as good as new (well minus the warranty of course) :)

Intel's i3/5/7-series CPUs don't have pins anymore...

I see, they are now LGA type which is nice for 2nd hand CPUs but what if you broke one on the pins on the socket? A new mobo, that sound terrific, anyway my only Intel CPU is inside the laptop and I hope I'll never have to open up that one :)

Well, a new motherboard is usually cheaper than a new CPU, so...

Yeah, but its PIA to install :D

I don't think that a used chip is a bad idea per se. But it implies some risks as you never know what that chip went through in its previous life. For all you know, it could have been barely used, with little exposure to thermal stress, or used day and night under thermal stress.

Note however that a competent cooler is a way to protect and extend the life of the CPU. So, even a budget, a correct cooler with a good thermal paste makes sense. I would not use the stock cooler with an I7, because it comes with a very basic thermal paste and tends to get easily clogged by dust, so will, with time, loose its cooling perforamnce.

A Coolermaster Hyper 212, or an Artic Freezer 13 are dependable and reasonably priced, and more than enough for the I7 if there is no intention to overclock. For few bucks less, the hyper TX or the Artic 7 are also doing they job well. I personnaly favour the Arctic 13, because it is very easy to mount it in an alreday built system.

You could always go with a closed-loop liquid CPU cooler. Then it's just a heatsink with an attached fan/radiator that gets put on the back or top of your case.