With regards to O/C'ing it and voiding the warranty (cooking the CPU) Intel offer a scheme that covers destroying your CPU over clocking it allowing you to RMA the chip for free. (1 time only, costs a few bucks 2nd time round).
O/C'ing it should have no influence on 2nd hand price unless you cook it.
I can't load the images you've posted on this computer but from the description take care when seating the heat sink. Make sure you don't use loads of TIM and preferably apply a very small amount in a cross shape across the IHS, making sure you don't put it right into the corners. (it will spread more than you think!) Try not to move the Thermalright HS around when you mount it. Put the case on it's side, apply the TIM then drop the Thermalright on top and don't wiggle it about!
What are your idle temps?
What version of P95 are you running? if it's 28.1 expect scorching temps as it runs AVX2, if 27.9 then it's AVX and your temps aren't anything out of the ordinary. Just remember that if you've not changed anything in the BIOS it will run in adaptive mode as default and will bump the core voltage up when running P95. Get HWMonitor/info installed and keep an eye on your core voltage. XTU is a reasonable stress tester and doesn't use AVX apart from when using XTU benchmark.
If you want to find out how good it is at O/C'ing go back into the CPU section and look for the Haswell O/C thread that Darkwizzle kindly put together. On page one there's a guide to get you started. Follow it without jumping all over the BIOS changing voltages at random and you should do just fine.
There's also a 3 step guide done by ASUS and another guide on Linus.Edited by Doug2507 - 9/27/13 at 4:52am