You buy on condition and inspection, road test perhaps, knowledge obtained from places like Briskoda where owners have owned the model and actually used it, in and out of warranty if the model is older enough. You can say which engine it has and find out if any engine have particular flaws, same for gearbox, suspension etc.. Of course you do need to be a lot more specific with details, if you put the actual car up and who's selling it where sometimes members might even know the car or actually looked at it for sale now or before.
Most on here (not me, I'm the opposite, why people buy modern VWs is almost beyond me) will perhaps be biased towards the good elements of the car and marque but there are so many threads and posts here to also find the issues.
As I put if you have the VIN some even have access to look up details including perhaps service history.
One thing to note is that last year VW UK finally fell in line with VW in the rest of Europe (the world) and admitted the rubber cambelts didn't need changing at 50K-miles/5yrs (to late for me and others).
I'd want to plug in a scan tool and see what error codes have been left or are on the vehicle but I expected you might have to do that out of sight of the sales people, it'd be part of any inspection check on a modern car, I'd work them into any negotiations, if negotiations are still available now.
To know how good any car is as an example of that model you'd probably need to drive a few others, preferably same engine, gearbox, suspension, age/mileage. Cinch style buying seem simple poke and hope buying which suits many - but you only have a short time to bounce it back if you've made a mistake.