Buying used parts is always a "crap shoot'. The reasoning is that depending on where you buy from (a store s the guy down the street) you don't know the condition of the parts you are purchasing. If buying from a store is the part from a "Demo" model that has just been on in the background or is it a return package that they are reselling? If it is from a person you know, how long have they had it and what type of usage/overclocking have they done?
Bottom line is most used parts will not carry any type of warranty and many manufacturer warranties are not always transferable from one person to the next. Some businesses may offer like a limited 90 day warranty, but to be honest these are fairly useless. Simply put when buying used parts there are no guarantees on what you are going to get. You don't know if the part has any intermittent issues and such.
Personally I would rarely buy a tonne of components used. I might be persuaded to buy a higher end video card under the right circumstances, for example, person just bought a GTX 770 and the new GTX 970 came out and they want to upgrade and are giving you a really good deal on the 770. I think you would be better suited buying a pre-built computer with full warranty or buying your parts new but selecting lower end items.
In the end it is up to you, but as the old axioms go, "you get what you pay for" and "buyer beware".
Good luck!