Buying used vs new?

I bought a "part" (actually a pair of parts) that was built in 1939, so I don't know about a maximum age.  The pair of compression drivers are among the best things I've added to my system.  I don't think there is any warranty remaining on it; I suppose that is the downside.

If the seller is reputable and will stand behind the sale, used is fine.

I recently purchased an ARC SP6 pre-amp from a reputable dealer, it had an issue, I had it fixed (not expensive) and they paid for it. 

The pre-amp is wonderful, and I will no longer need a $20,000 pre-amp, so new is not an option for me.  I am VERY happy with my used gear.

Cheers!

Buying used from a dealer? Dont know where product has been. Like meeting a girl or guy. 
How does one know about product history, ie preamp, or does it matter?

In general, who cares where it has been. Unless banged up or with grease dripping from it. Most likely bought by someone that knew something about high end audio. I guess if it is from an inner city pawn shop… I would be hesitant. 

The only thing I would add is that the age makes a big difference. You should figure on replacing the capacitors at somewhere between 15 and 20 years old. If the preamp is 15 years old (+/- 2005 vintage) you need to find out the cost to recap it and figure that into the cost. I spent $7K in the last two years recapping my Krell amp, preamp, phono preamp, and CD player but it was worth it because I intend to keep these pieces for the long term. My point is that recapping is pretty expensive on high end gear so you need to take that into account when buying used.