At what point do you...

You have to be careful buying used audio equipment. I have bought stuff and 2 ,3,6 months later it dies.probability from the same reasons the guy sold it....Buyer Beware.....

For me it depends on the component, the price, availability, and the dealer or person I’m buying from. I would never buy a CD transport used but will always buy cables used when available.  I bought my McIntosh Integrated used from a reputable dealer (Audio Classics) and many open box items with a full warranty.  For new, I look for huge discounts and close outs.  I purchased my Elac towers on close out for 50% off from Music Direct and recently bought a pair of Dynaudio bookshelf speakers for my office system at 25% off list. I never buy on impulse and always thoroughly research a product before pulling the trigger. 

Everything except my turntable and cartridge has been dealer demo or closeout. I like the peace of mind. My power line conditioner was also new (with a nice discount) since it wan't available with any savings used.   I did buy a used phono stage from someone on USAM, but spoke with him at length and since it has no moving parts or even external switches, I took the chance and it was in perfect condition as he said. Save about 25% over new. 

I buy open box and dealer demo all the time. I never buy used. I did get into trouble once. I bought a dealer demo Clearaudio Double Matrix and the guy put some god awful solution in it that eventually choked the fluid pump. Fortunately, Musical Surroundings covered it under warranty and let me install a new pump myself which saved me from shipping it across the country. When I took it apart the tubing and pump were full of a thick white residue. I cleaned it all out and replaced the filter. I also now know the machine backwards and forwards and can deal with anything that happens to it. 

Used, demo, open box mostly- it’s the only way I can afford targeted sonics.  New if used option not available such as a newer model.