Is it a terrible thing to sell a car with an undisclosed issue?

I've done this before, I did feel a bit bad but needs must. I didn't earn much money, and I'd had a succession of car issues. First a Ford KA, which my Dad bless him, one day accidently set fire too whilst he was underneath it welding (a spark went into the open door and set fire to the chairs) the car was a write off. Then I got a Form Puma, (old style!) and the engine just died one day, another write off. Then I got a Corsa (old style again) and everywhere I went I had to bring oil with me to top it up as it just kept leaking. So I'd come out of work and top the oil up that had leaked while I'd been at work sort of thing. I was putting off the bigger more expensive problem (that I suspect was also not mentioned when I bought the car). Eventually I knew I'd better get rid before it's another write off car that i'll only get scrap for. So I put an advert in auto trader, didn't mention the problems, did put sold as seen though. Few days later a 'vauxhall corsa loving expert' came over to buy it, I thought, ah crap if he loves corsas and is an expert, surely he'll notice stuff wrong with it on the test drive. I went on the test drive with him (in case he nicked it ha, big bloke verses young girl me) and he was like, yeah i'll take it. I sold it for asking price. I can't remember what was wrong with it now, but I just remember my Dad, who's pretty experienced with cars, was like, yeah this is a very expensive job to fix, you just need to get rid before it goes! It wasn't anything dangerous, just annoying and I thought, argh I am not in a position to be truthful this time sorry.