This reminds me of a talk I had at around age 17 with my Father on how to buy a 2nd hand car

.....needless to say I still can't get it right!
Okay, this I can give you a little bit of advice on!. First of all, like any form of D.J equipment the cosmetic appearance give some idea of the use (or abuse) that its had. If its scratched and bashed about then its obviously not been looked after and there could be internal damage so don't waste your money if it looks like it's been through a waste disposal unit or thrown down a flight of stairs.
Other General Tips before buying a 2nd Hand Mixer:-
(1) Check for fader problems / wear. The easiest way to do this is to crank the mixer & amp upto a high level with no music signal present. Move each channel fader and any master fader(s) up and down listening for any thumps, bangs or crackles as the fader is being moved. Lower the volume levels and play some music through each channel, raising the fader slowly to increase the volume. Check that the levels increase smoothly with no interference of drop out as the fader moves.
(2) Repeat the test above, with any tone controls or EQ and the cross-fader. Cross faders are consumable items and likely to wear. If you come across a X-fader which needs replacing, then if the mixer is cosmetically okay you should still consider going ahead with the sale, but negotiate a discount in order for you to replace the X-Fader
(3) Check the headphone Jacks and the rear panel signal in / out connections for damage / poor contact by
GENTLY wiggling them whilst they are connected. If you hear crackles then this indicates a poor contact with the internal PCB, which will become worse and cause problems in time.
(4) Ask around and search this forum for any reviews or complaints about that product. Other D.J's are likely to be the best form of advertisement for any product.
(5) Trust your instincts - if something doesn't appear to be right then it possibly isn't!
(6) Get a bargain - most private sellers, advertise goods at slightly above what they would accept for it, with the anticipation that they are likely to get "beaten down" in price. Honour this, by offering them $10 - $15 below the asking price. Be postitive and assertive in your bargaining and phrase the offer as "I'll give you $400 for it" rather than "Will you accept $400?", by phrasing it as a bold statement rather than a question you remove the obligation of a directly replied "yes / no" answer