I have always bought from car supermarkets, although there don't seem to be so many of those these days. They don't undertake to fix them if something goes wrong immediately, but you can return them, within a short period. They don't want to have to take them back so they seem to make sure they're in good condition before they sell them.
The last two cars we bought as a family we've used Cazoo and been very pleased with them. I don't know what the situation is with the company at the moment; they still seem to be going, but the cars on their site all mention a dealer, so perhaps they are just brokers now. Could be worth looking into.
During Covid we had to buy a car online; not being allowed to visit a showroom. Their salesman went all round the car video-ing it and sent that to us. When we decided to buy they drove it over and we had 14 days to reject it if we wished, as we hadn't seen it previously. It was some distance away, so quite an incentive to make sure they delivered a car in good condition.
We've been very satisfied with every car we've bought. No hard sell any time; you just pick the car you fancy online, then make an appointment to see them. We usually go for a couple of years old and spend under £10K (might have to increase since Covid). Cazoo made a very reasonable part-exchange offer on the old car, and no hassle just to drive to their depot in one and drive away in the other.
Over the last 30 years as a family we've had two Fiats and several Vauxhalls. I know they are not considered 'the Thing' on Mumsnet, but we've never had a moment's trouble with any of them. I think because they are churned out in their thousands, and Vauxhalls, in particular have many parts in common across their range, they are easy to fix and obtain spares should a problem arise.