Buying used vs new?

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.

If you can live at a few dings, there is no reason not to buy used speakers unless they are decades old. But even in the case of needing to recone old drivers, that is not a big deal.
When buying used speakers, best to buy ones that you know are to your liking. Great to hear them first but reviews can be a guide post.
Two things to not buy used are TT and cartridges. Obviously, these things wear out. I bought a used Woodsong Garrard 301. It was in great shape but still needed a couple of hundred to make it functional. But you really can’t trust a retipped cartridge. New is always best. SS components are usually fine used. Tubes do need replacing and the heat from tubes can effect the longevity. I guess this can also be said about Class A amps. But, mostly, preamps and amps are good used. And CD players and transports also wear out but quality ones last decades. I have a 25 year old Theta Compli transport that is still great. Best to have transferable warranties. There is always a chance that a unit will fail but if defective it usually does so in the first year or less. Usually safe with major brands.

I guess I have been very lucky when it comes to used equipment. Touch wood! I would say almost 60% of my gear was bought used. The only time I bought something that broke after a few month was the Naim Unitiqute integrated amp. But generally, I never buy anything older than 5 years.