Most Vectrexkäufe should probably be done on ebay, because on other platforms as Quoka or hood, the supply of very thin Vectrexsachen.
The descriptions and images of ebay sellers are in this case very different. One can now as a prospective customer to put together a long list of questions and send it to the ebay seller. One should there but better focus on the key issues:

• Does the Vectrex and / the Controller 100%?
• What is the overall condition of the Vectrex? Already once opened the Vectrex?

Why these questions? What else should I note otherwise?

• The controller may be the weakest component of the Vectrex: Depending on the use of individual keys no longer work or a return spring on the device is broken. Is on images of the joystick to see in slate position, it is clear, that the return spring is broken.
• hum Most Vectrexmodelle, this is completely normal, yes the hum heard almost Vectrex and has its basis in a design flaw, which was modified only in the last shipped Vectrexkonsolen.
  Electronics hobbyists can solve the hum problem itself. If you want necessarily a Vectrex, the not buzzing from manufacturer, then you have to pay attention to the serial number: All Vectrex consoles, whose serial number with 31… begin, not hum. One should note, however,, that a few modules, such as the 72-in-1 Multicart by Andy Coleman on these Vectrexmodellen not run.
• one white from the outset already, that it is left not only in the console Vectrex, but even games will be added, then you should for an offer of a console with as many games and especially keep two controllers out. Bottom line, you come away so much cheaper.

They are aware of only a few questions and information, so that the biggest surprises should be prevented. One can not imagine many other question, but must consider, if you really want to maltreat a seller with a long list of questions. And some of it is anyway subjectively, so one saves for example the question of a sharp image better; especially since the sharpness is highly dependent on the adjusted brightness.

So rather provide only the most important issues. And: Most offered Vectrexkonsolen are fine.

addendum on 1. December 2013: As I have to say, currently almost exclusively on ebay “Buy immediately”-Deals with plenty overestimated prices. Then you should not get involved and waiting for suitable offers and the sale itself. An alternative to the sale are classified, even from ebay themselves, but then also at Quoka, Kalaydo or how the providers are called. But it takes a little patience or a thick wallet. More about the prices can be found in the post “What is the price of a Vectrex? What the Games?“.