I've had good luck with Chinese sources on Ebay -for used Rubidium frequency references, GPS receivers and antennas - but I was careful to buy only from top-rated sellers.
I've bought used test equipment on Ebay (Agilent, Tektronix), but only from top-rated US and EU sellers. It's a cliche, but it's a cliche for a reason: if it's too good to be true, it probably is.
For components that need to be reliable (about 100% of my needs), I stick to mainstream distributors like Digi-key, Mouser, Element14, etc. Asian supply chains are rife with counterfeit parts, so I rely on the major distributors to police their own stock. Fake, unreliable parts simply cost me too much, in time and money.
Someone mentioned Jaycar. Recently, I walked into a Jaycar store looking for D-sub connectors, for a test and measurement setup. The quality on offer was so bad that I walked out without buying anything, price be damned. I refuse to use <expletive> <expletive> components in anything I do.