buying used valves

I am currently building a forged 2JZ for a 800-1000 hp project. I've got a set of used valves which I bought from someone and was wondering if I can use them or should I buy new ones? Visually the valves seem fine and the stem diameter is within spec. However, I have not inspected the item under a microscope.

hello, i would never use s/hand valves if i don't know their history and age, if you can save for a while longer to buy new ones, a few weeks more saving is better than a few years saving to rebuild your engine again

As Ross said, unless you KNOW their history (and even then) it' s a bit of a crap shoot - they may be perfectly fine, or they may have been overheated and/or be developing fatigue cracks and so be liable to failure. The alloys used may not suit the application, as some aftermarket ones are available in several different materials, with the cheapest being good for replacement and there being much more expensive options for more extreme conditions.

It isn't unusual for a valve failure to take out a complete engine, especially if the failure is around peak rpm and full load, and that's going to cost a lot more than a good quality* set of OEM or aftermarket valves.

*I say "good quality" as there're some brands that seem to have a disproportionate number of failures, apparently fatigue cracking at the transition from stem to head which has led to the head breaking off. I suspect some of the failures may have been due to insufficient spring control over the valves leading to the valves physically bouncing on the seats, as that's really the only way to get the very high loads required for the damage to occur.