There are many scammers out to steal your money by listing a quality quad bike for give away money. If it seems to cheap to be true then it probably is a fake.
Most of these are buy it now for £1500 or pay by wire transfer or cheque and the seller has zero feedback and has recently registered,many as business sellers.
Ask to view the machine before buying and if they are away on "vacation" so viewing is not possible then you know it is a scam.
Dont be fooled by cheap chinese quads as these are death traps that make unbelievable claims about power and reliability but fall to bits and then prove costly to keep replacing parts or you find that parts are not available.
The brand makes cost more but yamaha,honda,suzuki,polaris,can-am and qz brand are built well and last.
Dont buy a agricultural registered quad claiming its road legal because they are not (unless you are a farmer) and then they have very restricted use. The only quad that is fully road legal is registered as plg (private light goods) regardless of what claims the seller makes.
If you want to know more then look on quadheaven . co. uk as all the above information can be found within their forums and many friendly members can give you advice about all your quad biking needs.