Hello there,
I have recently purchased an old Prescott CPU myself and must say that it can be a very good deal depending on the use you have in mind for it and the hardware you already have.
If you (as in my case) have an old ASUS socket 478 main board and want to update your specs and extend its useful life a little, then it is probably a good business considering your needs.
Regarding the Prescott discussion, although it is not the best CPU for OC it is still possible to get some extra HP out of them. For instance, I had a Prescott P4 2,8G with 533MHz bus and 512KB cache (no HT) that I used to overclock to 3.5GHz with very acceptable temperatures (using a Zalman cooler at its slowest velocity).
Now, I have recently purchased a 3.0GHz, 1MB cache CPU with 800MHz bus (with HT), exactly as the one Wobbly purchased (SL8JZ) and have overclocked it to 3.9GHz with idle temperatures of 49ºC while I am writing this posting. I am using the Zalman cooler on the slower velocity (which is probably equivalent to the Intel original cooler for this CPU I guess) and my room temperature is around 20ºC. Let me say I am very happy with my purchase.
Just in case you are interested, here are my basic PC configuration and overclocking settings:
ASUS P4P800SE (Intel 865PE chipset) with PAT enabled (as in the 875 chipset, thanks to ASUS engineers).
Dual Channel configured Kingston DDR400 RAM (2 x 1024MB) @ 416MHz (832MHz).
RAM Timings: 2.5-3-3-6
Prescott P4 3.0GHz 800MHz (SL8JZ) @ 3.9GHz 1040MHz (CPU bus at 260MHz, RAM bus at 320MHz)
Zalman CNPS700B-AlCu CPU cooler (not the best one, but better than the original Intel cooler)
CPUIdle Extreme software running all the time.
Regarding the marks on your CPU, I too agree that some OEM new CPUs may have marks on them and that does not mean they are used. In my case I purchased it in new old stock condition from a very good friend's shop (whom I deeply trust) and it also had little marks on it due to prior testing of the CPU to check that it worked fine.
IMHO you did well with your purchase if you know how to squeeze its potential with a little OC and without changing the rest of your hardware.
Best regards,
Plinio.