In fairness, a house dropping by 30% in value would have to be a pretty substantial crash in the market!
But if you’re asking me as a general principle then yes, I don’t think you should renegotiate once you’ve agreed a price.
As I said, would you be happy with the reverse? The market increases and at the last minute the seller demands an extra 10%, 20%, 30% (to use your figures). I imagine most buyers would be extremely upset after they had made and agreed on an offer to suddenly be expected to pay a hefty chunk more,
The property market will always fluctuate - and in the long run it will always increase. I bought at the very peak of the market before the global crash 16 years ago. Even worse I had a 100% mortgage. My property has just sold for about 35% more than I paid for it. And I priced it very competitively- I could have probably got an extra 10% if I was willing to wait (speed was a priority for me).
If you’re trying to flip property then yes, you might have an issue but if it’s a longer term move then forget about the value. But a house you love that you can comfortably afford and enjoy your home in the following years. Despite what doom merchants like to predict, property values will always steadily increase in the long term. It’s about riding about short term dips.
So to return to your original question again - if you can truly and honestly say that you’d happily pay extra if the sellers asked you to (in a rising market) then fair enough. But the vast majority of people would be very very pissed off to be asked for more money after previously agreeing a price solely due to markets climbing. The principle cuts both ways. It’s a shitty move.
To use your analogy, you buy a Hoover online at Currys but when you go to pick it up they say you have to pay an extra £50 because that’s what Argos is charging. Would you be happy to pay up the extra? If you wouldn’t pay the extra then you shouldn’t expect money off either. You honour the agreement you made - and that applies to both sides.
The only exception for me would be if the other party had unreasonably delayed things. Then I would say that a renegotiated price is the consequence of their poor behaviour.