Most of my equipment was purchased used.
Research: read, read, and read, then read more; knowledge is power.
Create a list of what you are looking for, and begin to peruse the market with the objective of learning the nature of the market itself.
(Incidentally, you will find that some sellers have not researched the market; they say to themselves, ‘I bet I could get x-amount of dollars for this’, and sell at a low price: good deals to be found there.)
Over time, you will begin to perceive the general fair market price, and learn to identify the ‘estate sales’ types who just want to make a buck vs the true audiophile-owned items that have been valued and treated well.
My rules are:
The seller must include a statement of condition that speaks to an audiophile’s concerns - none of this, ‘tested, works, look at the pictures.’
Many times I’ve seen exactly what I’ve been looking for at an attractive price, but no statement of condition, and if the seller can’t be bothered to write a few thoughtful lines, then I do not trust the seller.
If it says, ‘reconditioned’ or ‘restored’, the seller must describe the alterations in detail and must include proof of them in the ad itself.
(Though, I do not buy ‘reconditioned’ or ‘restored’ equipment - I want the original.)
Blurry or too-dark photos, most likely a scam.
If there is any indication of lying whatsoever, or even of glossing over the truth, I’m gone.
There is a certain type of audiophile who values his gear and treats it well, and wishes it will go to someone who will respect it as he did - you can tell who they are, the best persons to buy from, but that takes reading many ads to discern who they are.
Honest folks display their honestly; it’s part of who they are and their honesty is apparent.
Dishonest folks leave you guessing about things.
The most important rule: always trust your intuition; the only times I’ve been burned is by ignoring my intuition and being too hopeful.
Best of luck to you, aamiransari.