Interesting - never heard of that website, but would have probably had a look if I’d known about it.
Before calling I would make sure I’ve read up on common faults with the specific models you’re looking at and check for any reviews of the dealers online (google and autotrader are reasonable places to start with that) so you’re prewarned of any possible pitfalls.
If you have the registration plates you may want to run a car check as well which will give you MOT history, let you know if there’s outstanding finance or it’s previously been written off.
Other than that, you want to arrange to both see and test drive it. Make it clear that you’ve got a shortlist of several you need to see so they don’t try too hard to pressure you into buying immediately. Make sure they’ve got the right paperwork (service book, manual, V5C etc), check the condition of everything looks sound (it’s hard to tell if like me you’re not an expert, but I there was a missing spare wheel on one car a couple of weeks ago and duct tape holding a mirror onto another…) and dents may be a sign of previous minor unreported accidents.