RAC car data checks are £25 for 5 checks, which is pretty good value, i HPI'd an 11,000 mile diesel corsa on a 56 plate that was being sold privately for around £4,200, something sounded too good to be true as this car was far cheaper than much older diesel corsa's, it turns out there was still finance owing, there was a note saying something about 100,000 miles and the owner before this person selling it had only kept it for 4 months.
No mention of any of this in the Advert, why is the car advertising website not picking this up, they give plenty of warnings but do nothing themselves.
HPI - RAC, get 5 checks for £25
Car Tax Online - you can check when the car was registered, if the tax has run out, etc.
ask mid - check to see if the car is on the insurance database
MOTINFO - check a cars MOT records online, need reg and either V5 ref or MOT certificate number