Not buyer's remorse but regret, 2 in respect of hifi:
1) XP5XS because the difference it brought was so small as to not be worth the cost - it lasted a month before I sold it, at a loss.
2) ND5XS - though the regret was not about buying it, as it was a perfectly good player that I did enjoy for a while, rather it was regret that I hadn't done more research about the new (to me) subject of streaming, only afterwards learning more and discovering that for the same money I could have got even better sound quality.
Outside of hifi, I have made a couple of bad financial decisions, that caused far more serious regret than anything hifi, with losses that could have bought a truly top-end hifi system: I would have had buyer's remorse had I spent that amount of money on hifi, real guilt at the commitment of such large resources purely for my own hobby - but I would still have it to enjoy every day for the rest of my life. ...now that does twist the regret!
On the subject of cars, in general I have never understood people buying new cars, as you lose so much on the day you buy them - and if simply because of having enough money to not be concerned, that only makes sense if it is the very best car you could possibly want, otherwise a secondhand one gets you better for the same money, yet some people buy their new mundane motors with amazing regularity. (Of course, someone has to, otherwise there'd be no secondhand models for others!) Neither have I understood the fixation some people have on having to keep the semblence of new, buying again every year or so, but that is a different subject....