My recommendation for your first one or two guns is to buy new. Then buy a complete gearbox that works to have spare parts as well as to learn the inner workings of your gun so you will be able to maintain it and upgrade/replace parts as they break. I would recommend starting with a gun with a v2 gearbox, it's the most commonly used (the v3 being the next most common). In general all gearboxes have the same parts (for the most part) just laid out slightly differently. I started with a v6 and v2. The v2 was by far the easier of the two to learn on.
Now to answer your question,
The more info the owner can give on the gun the better. Parts used, how many rounds/games the gun has seen in its current condition, how many rounds the gun has seen as a whole, what might need replacing in the near future, any issues with the gun, wether mechanically or aesthetically, the velocity, when it was last chronoed, when it was last maintained, wether it's properly shimmed by a reputable tech or not, what bbs have been used (a definite indicator if a barrel change is needed), what batteries have been used, (will tell you if trigger contacts are on their way out or not), if it has a MOSFET, and how many owners the gun has had.
mk43, ump.45, p90, barret, m14, g33, k98, m4 tac, m4 swat, mp5 navy, vsr10, p226, m9.